Fix pgpool crash when pgpool child process exits.

When a pgpool child process exits, close_all_backend_connections() is
called, which is responsible for closing all connections to backend in
the connection pool. It used mistakenly MAIN_CONNECTION macro, which
is fine for current active connections but is not good for pooled
connections because a main node could be different at the time when
the connection pool was created. Fix is using in_use_backend()
instead.

Reported-by: Emond Papegaaij
Backpatch-through: v4.2

src/protocol/pool_connection_pool.c

@@ -1073,11 +1073,15 @@ close_all_backend_connections(void)
 
 	for (i = 0; i < pool_config->max_pool; i++, p++)
 	{
-		if (!MAIN_CONNECTION(p))
+		int	backend_id = in_use_backend_id(p);
+
+		if (backend_id < 0)
 			continue;
-		if (!MAIN_CONNECTION(p)->sp)
+		if (CONNECTION_SLOT(p, backend_id) == NULL)
 			continue;
-		if (MAIN_CONNECTION(p)->sp->user == NULL)
+		if (CONNECTION_SLOT(p, backend_id)->sp == NULL)
+			continue;
+		if (CONNECTION_SLOT(p, backend_id)->sp->user == NULL)
 			continue;
 		pool_send_frontend_exits(p);
 	}