MDEV-6040: MariaDB hangs if terminated quickly after start

We need to use mysql_cond_broadcast() rather than _signal for
COND_thread_count, as there can be multiple waiters.

Thanks to Pavel Ivanov for reporting both the problem and the
solution.

sql/slave.cc

@@ -318,7 +318,7 @@ handle_slave_init(void *arg __attribute__((unused)))
 
   mysql_mutex_lock(&LOCK_thread_count);
   slave_init_thread_running= false;
-  mysql_cond_signal(&COND_thread_count);
+  mysql_cond_broadcast(&COND_thread_count);
   mysql_mutex_unlock(&LOCK_thread_count);
 
   return 0;