Fix crash and invalid write reported by valgrind for 'test_ps_async-t'

[JavierJF]

• A clear description of the issue

During testing several crashes was encountered while test 'test_ps_large_result-t' was being executed, crash could be reproduced locally successfully:

Error: signal 11:
./src/proxysql(_Z13crash_handleri+0x34)[0x558ae2e6fbd3]
/usr/lib/libc.so.6(+0x3cf80)[0x7f2c59d31f80]
./src/proxysql(_ZN15MySQL_ResultSet8add_row2EP13st_mysql_rowsPh+0x27)[0x558ae2f6a62d]
./src/proxysql(_ZN15MySQL_ResultSet14init_with_stmtEv+0x21f)[0x558ae2f69ec3]
./src/proxysql(_ZN13MySQL_Session31MySQL_Stmt_Result_to_MySQL_wireEP13st_mysql_stmtP16MySQL_Connection+0xa2)[0x558ae2f4f1c8]
./src/proxysql(_ZN13MySQL_Session35handler_rc0_PROCESSING_STMT_EXECUTEEP17MySQL_Data_Stream+0x61)[0x558ae2f40e39]
./src/proxysql(_ZN13MySQL_Session7handlerEv+0x106b)[0x558ae2f43453]
./src/proxysql(_ZN12MySQL_Thread20process_all_sessionsEv+0xeb8)[0x558ae2f10f9a]
./src/proxysql(_ZN12MySQL_Thread3runEv+0x14e6)[0x558ae2f0f020]
./src/proxysql(_Z24mysql_worker_thread_funcPv+0xca)[0x558ae2e6482e]
/usr/lib/libpthread.so.0(+0x9299)[0x7f2c5a235299]
/usr/lib/libc.so.6(clone+0x43)[0x7f2c59df4053]
 ---- ./src/proxysql(_Z13crash_handleri+0x34) [0x558ae2e6fbd3] : crash_handler(int)
 ---- ./src/proxysql(_ZN15MySQL_ResultSet8add_row2EP13st_mysql_rowsPh+0x27) [0x558ae2f6a62d] : MySQL_ResultSet::add_row2(st_mysql_rows*, unsigned char*)
 ---- ./src/proxysql(_ZN15MySQL_ResultSet14init_with_stmtEv+0x21f) [0x558ae2f69ec3] : MySQL_ResultSet::init_with_stmt()
 ---- ./src/proxysql(_ZN13MySQL_Session31MySQL_Stmt_Result_to_MySQL_wireEP13st_mysql_stmtP16MySQL_Connection+0xa2) [0x558ae2f4f1c8] : MySQL_Session::MySQL_Stmt_Result_to_MySQL_wire(st_mysql_stmt*, MySQL_Connection*)
 ---- ./src/proxysql(_ZN13MySQL_Session35handler_rc0_PROCESSING_STMT_EXECUTEEP17MySQL_Data_Stream+0x61) [0x558ae2f40e39] : MySQL_Session::handler_rc0_PROCESSING_STMT_EXECUTE(MySQL_Data_Stream*)
 ---- ./src/proxysql(_ZN13MySQL_Session7handlerEv+0x106b) [0x558ae2f43453] : MySQL_Session::handler()
 ---- ./src/proxysql(_ZN12MySQL_Thread20process_all_sessionsEv+0xeb8) [0x558ae2f10f9a] : MySQL_Thread::process_all_sessions()
 ---- ./src/proxysql(_ZN12MySQL_Thread3runEv+0x14e6) [0x558ae2f0f020] : MySQL_Thread::run()
 ---- ./src/proxysql(_Z24mysql_worker_thread_funcPv+0xca) [0x558ae2e6482e] : mysql_worker_thread_func(void*)
[1]    29078 segmentation fault (core dumped)  ./src/proxysql -f -c  -D single_datadir

Attached coredump and others drove investigation towards test_ps_async-t when valgrind reported a invalid writer error:
valgrind-out.txt

This error should be fixed and the parallel implementation of inside ProxySQL for which this test was a 'POC' shall be patched as well.

• ProxySQL version

v2.1.1

• OS version

NAME="Arch Linux"
PRETTY_NAME="Arch Linux"
ID=arch
BUILD_ID=rolling

• The steps to reproduce the issue

Run test_ps_async-t under valgrind.

• Coredump

crash_coredump.zip