Avoid crash in MySQL_Result_to_MySQL_wire without myds

[renecannao]

• A clear description of the issue

It has been detected a crash in MySQL_Result_to_MySQL_wire with myds is NULL

• ProxySQL version

2.1.0 and 2.1.1

• OS version

Any

• The steps to reproduce the issue

Not sure how to reproduce it

• The full ProxySQL error log (default location: /var/lib/proxysql/proxysql.log)

Full log not available, but this is a backtrace from 2.1.0:

2021-03-22 20:49:43 MySQL_Session.cpp:3847:handler_minus1_ClientLibraryError(): [ERROR] Detected a broken connection during query on (0,10.1.5.174,3306,1859974) , FD (Conn:54 , MyDS:54) : 2014, Commands out of sync; you can't run this command now

Error: signal 11:

/usr/bin/proxysql(_Z13crash_handleri+0x2a)[0x58ea0a]

/lib64/libc.so.6(+0x36400)[0x7f99e3596400]
/usr/bin/proxysql(_ZN13MySQL_Session26MySQL_Result_to_MySQL_wireEP8st_mysqlP15MySQL_ResultSetP17MySQL_Data_Stream+0x1e3)[0x5ead93]
/usr/bin/proxysql(_ZN13MySQL_Session7handlerEv+0x7a1)[0x5fe241]
/usr/bin/proxysql(_ZN12MySQL_Thread20process_all_sessionsEv+0x3bc)[0x5dad0c]
/usr/bin/proxysql(_ZN12MySQL_Thread3runEv+0x70b)[0x5e223b]
/usr/bin/proxysql(_Z24mysql_worker_thread_funcPv+0x6c)[0x5874fc]
/lib64/libpthread.so.0(+0x7ea5)[0x7f99e4776ea5]
/lib64/libc.so.6(clone+0x6d)[0x7f99e365e8dd]
 ---- /usr/bin/proxysql(_Z13crash_handleri+0x2a) [0x58ea0a] : crash_handler(int)
 ---- /usr/bin/proxysql(_ZN13MySQL_Session26MySQL_Result_to_MySQL_wireEP8st_mysqlP15MySQL_ResultSetP17MySQL_Data_Stream+0x1e3) [0x5ead93] : MySQL_Session::MySQL_Result_to_MySQL_wire(st_mysql*, MySQL_ResultSet*, MySQL_Data_Stream*)
 ---- /usr/bin/proxysql(_ZN13MySQL_Session7handlerEv+0x7a1) [0x5fe241] : MySQL_Session::handler()
 ---- /usr/bin/proxysql(_ZN12MySQL_Thread20process_all_sessionsEv+0x3bc) [0x5dad0c] : MySQL_Thread::process_all_sessions()
 ---- /usr/bin/proxysql(_ZN12MySQL_Thread3runEv+0x70b) [0x5e223b] : MySQL_Thread::run()
 ---- /usr/bin/proxysql(_Z24mysql_worker_thread_funcPv+0x6c) [0x5874fc] : mysql_worker_thread_func(void*)