avoid rearrangeArguments decref uninitialized memory

from_cpython/Lib/test/test_string.py

-# expected: reffail
-# - unknown segfault
 import unittest, string
 from test import test_support, string_tests
 from UserList import UserList

src/runtime/descr.cpp

@@ -313,7 +313,7 @@ Box* BoxedMethodDescriptor::tppCall(Box* _self, CallRewriteArgs* rewrite_args, A
 
     Box** oargs = NULL;
 
-    Box* oargs_array[1];
+    Box* oargs_array[1] = { NULL };
     if (paramspec.totalReceived() > 3) {
         assert((paramspec.totalReceived() - 3) <= sizeof(oargs_array) / sizeof(oargs_array[0]));
         oargs = oargs_array;