Bug#58080: Crash on failure to create a thread to handle a user connection

The problem was that the scheduler function used to handle a
new user connection could use the ER() macro without having a
THD object bound to the current thread. The crash would happen
whenever the function failed to create a new thread to handle a
user connection. Thread creation can fail due to lack or limit
of available resources.

The solution is to simply use the ER_THD() macro instead and pass
to it the THD object which would be bound to the connection.

Fix was tested manually. In a test case, it is too cumbersome to
inject a error in this context.

sql/mysqld.cc

@@ -4949,7 +4949,7 @@ void create_thread_to_handle_connection(THD *thd)
       statistic_increment(aborted_connects,&LOCK_status);
       /* Can't use my_error() since store_globals has not been called. */
       my_snprintf(error_message_buff, sizeof(error_message_buff),
-                  ER(ER_CANT_CREATE_THREAD), error);
+                  ER_THD(thd, ER_CANT_CREATE_THREAD), error);
       net_send_error(thd, ER_CANT_CREATE_THREAD, error_message_buff, NULL);
       mysql_mutex_lock(&LOCK_thread_count);
       close_connection(thd,0,0);