COM_EXECUTE packet" and #2795 "prepare + execute without bind_param crashes
 server" and #2473 "seg fault running tests/client_test.c": 
- length checking added to packet parser 
- default impelemntation of Item_param::set_param_func will work in
case of malformed packet.
No test cases are possible in our test suite, as there are no tests 
operating on protocol layer.