MXS-1977: Fix protocol and readwritesplit memory leaks

The protocol could leak memory in rare cases where several commands were
queued at the same time. Readwritesplit also didn't free the memory it
acquired via qc_get_table_names.

server/modules/protocol/MySQL/mysql_common.cc

@@ -637,8 +637,9 @@ void protocol_remove_srv_command(MySQLProtocol* p)
     }
     else
     {
-        p->protocol_command = *(s->scom_next);
+        server_command_t tmp = *(s->scom_next);
         MXS_FREE(s->scom_next);
+        p->protocol_command = tmp;
     }
 }