Similarly to situation described in dcf4ebd1 (libgolang: Fix chan.close
to dequeue subscribers atomically), select can be also accessing a
channel object at the time of wakeup when that channel could be already
destroyed: select queues waiters to channels recv/send queues and upon
wakeup needs to dequeue them. This requires locking channels, not to
mention that a channel destroy with non-empty subscribers queue will
trigger bug panic.

Contrary to the fix for recv, we cannot rework select not to access
channel objects after wakeup, because for select upon wakeup all queued
channels could be already destroyed, not only selected one. Thus the fix
here is to incref/decref the channels for the duration where we need to
access them.

The bug was not caught by existing tests and was noted while doing
libgolang.cpp review for concurrency issues. With added test (hereby fix
is served by a bit amended _test_close_wakeup_all) the bug, if not
fixed, renders itself as e.g. the following under TSAN:

WARNING: ThreadSanitizer: data race (pid=4421)
  Write of size 8 at 0x7b1400000650 by thread T9:
    #0 free ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:649 (libtsan.so.0+0x2b46a)
    #1 free ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:643 (libtsan.so.0+0x2b46a)
    #2 golang::_chan::decref() golang/runtime/libgolang.cpp:479 (liblibgolang.so.0.1+0x4822)
    #3 _chanxdecref golang/runtime/libgolang.cpp:461 (liblibgolang.so.0.1+0x487a)
    #4 golang::chan<int>::operator=(decltype(nullptr)) golang/libgolang.h:296 (_golang_test.so+0x14cf9)
    #5 operator() golang/runtime/libgolang_test.cpp:304 (_golang_test.so+0x14cf9)
    #6 __invoke_impl<void, __test_close_wakeup_all(bool)::<lambda()>&> /usr/include/c++/8/bits/invoke.h:60 (_golang_test.so+0x14cf9)
    #7 __invoke<__test_close_wakeup_all(bool)::<lambda()>&> /usr/include/c++/8/bits/invoke.h:95 (_golang_test.so+0x14cf9)
    #8 __call<void> /usr/include/c++/8/functional:400 (_golang_test.so+0x14cf9)
    #9 operator()<> /usr/include/c++/8/functional:484 (_golang_test.so+0x14cf9)
    #10 _M_invoke /usr/include/c++/8/bits/std_function.h:297 (_golang_test.so+0x14cf9)
    #11 std::function<void ()>::operator()() const /usr/include/c++/8/bits/std_function.h:687 (_golang_test.so+0x1850c)
    #12 operator() golang/libgolang.h:273 (_golang_test.so+0x1843a)
    #13 _FUN golang/libgolang.h:271 (_golang_test.so+0x1843a)
    #14 <null> <null> (python2.7+0x1929e3)

  Previous read of size 8 at 0x7b1400000650 by thread T10:
    #0 golang::Sema::acquire() golang/runtime/libgolang.cpp:168 (liblibgolang.so.0.1+0x413a)
    #1 golang::Mutex::lock() golang/runtime/libgolang.cpp:179 (liblibgolang.so.0.1+0x424a)
    #2 operator() golang/runtime/libgolang.cpp:1044 (liblibgolang.so.0.1+0x424a)
    #3 _M_invoke /usr/include/c++/8/bits/std_function.h:297 (liblibgolang.so.0.1+0x424a)
    #4 std::function<void ()>::operator()() const /usr/include/c++/8/bits/std_function.h:687 (liblibgolang.so.0.1+0x5f07)
    #5 golang::_deferred::~_deferred() golang/runtime/libgolang.cpp:215 (liblibgolang.so.0.1+0x5f07)
    #6 __chanselect2 golang/runtime/libgolang.cpp:1044 (liblibgolang.so.0.1+0x5f07)
    #7 _chanselect2<true> golang/runtime/libgolang.cpp:968 (liblibgolang.so.0.1+0x6665)
    #8 _chanselect golang/runtime/libgolang.cpp:963 (liblibgolang.so.0.1+0x6665)
    #9 select golang/libgolang.h:386 (_golang_test.so+0x14fc1)
    #10 operator() golang/runtime/libgolang_test.cpp:320 (_golang_test.so+0x14fc1)
    #11 __invoke_impl<void, __test_close_wakeup_all(bool)::<lambda()>&> /usr/include/c++/8/bits/invoke.h:60 (_golang_test.so+0x14fc1)
    #12 __invoke<__test_close_wakeup_all(bool)::<lambda()>&> /usr/include/c++/8/bits/invoke.h:95 (_golang_test.so+0x14fc1)
    #13 __call<void> /usr/include/c++/8/functional:400 (_golang_test.so+0x14fc1)
    #14 operator()<> /usr/include/c++/8/functional:484 (_golang_test.so+0x14fc1)
    #15 _M_invoke /usr/include/c++/8/bits/std_function.h:297 (_golang_test.so+0x14fc1)
    #16 std::function<void ()>::operator()() const /usr/include/c++/8/bits/std_function.h:687 (_golang_test.so+0x1850c)
    #17 operator() golang/libgolang.h:273 (_golang_test.so+0x183da)
    #18 _FUN golang/libgolang.h:271 (_golang_test.so+0x183da)
    #19 <null> <null> (python2.7+0x1929e3)

  Thread T9 (tid=4661, running) created by main thread at:
    #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:915 (libtsan.so.0+0x2be1b)
    #1 PyThread_start_new_thread <null> (python2.7+0x19299f)
    #2 _taskgo golang/runtime/libgolang.cpp:123 (liblibgolang.so.0.1+0x3f98)
    #3 go<__test_close_wakeup_all(bool)::<lambda()> > golang/libgolang.h:271 (_golang_test.so+0x16c94)
    #4 __test_close_wakeup_all(bool) golang/runtime/libgolang_test.cpp:298 (_golang_test.so+0x16c94)
    #5 _test_close_wakeup_all_vsselect() golang/runtime/libgolang_test.cpp:342 (_golang_test.so+0x16f64)
    #6 __pyx_pf_6golang_12_golang_test_24test_close_wakeup_all_vsselect golang/_golang_test.cpp:4013 (_golang_test.so+0xd92a)
    #7 __pyx_pw_6golang_12_golang_test_25test_close_wakeup_all_vsselect golang/_golang_test.cpp:3978 (_golang_test.so+0xd92a)
    #8 PyEval_EvalFrameEx <null> (python2.7+0xf68b4)

  Thread T10 (tid=4662, running) created by main thread at:
    #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:915 (libtsan.so.0+0x2be1b)
    #1 PyThread_start_new_thread <null> (python2.7+0x19299f)
    #2 _taskgo golang/runtime/libgolang.cpp:123 (liblibgolang.so.0.1+0x3f98)
    #3 go<__test_close_wakeup_all(bool)::<lambda()> > golang/libgolang.h:271 (_golang_test.so+0x16d96)
    #4 __test_close_wakeup_all(bool) golang/runtime/libgolang_test.cpp:315 (_golang_test.so+0x16d96)
    #5 _test_close_wakeup_all_vsselect() golang/runtime/libgolang_test.cpp:342 (_golang_test.so+0x16f64)
    #6 __pyx_pf_6golang_12_golang_test_24test_close_wakeup_all_vsselect golang/_golang_test.cpp:4013 (_golang_test.so+0xd92a)
    #7 __pyx_pw_6golang_12_golang_test_25test_close_wakeup_all_vsselect golang/_golang_test.cpp:3978 (_golang_test.so+0xd92a)
    #8 PyEval_EvalFrameEx <null> (python2.7+0xf68b4)

and reliably crashes under regular builds.