Fix Pgpool-II child process crash during shutdown.

It is reported that pgpool child process crashes during shutdown.
[pgpool-general: 9261] Re: Segmentation fault during shutdown

The actual crash was in close_all_backend_connections().
close_all_backend_connections() was called because on_system_exit
registers child_will_go_down(). At the moment it seems pgpool child
had just started up and doing pool_init_cp(). The connection pool
object had not been completely initialized, that's cause of the crash.

To fix this, introduce a new static variable in child.c and set it
true when the connection pool object is initialized. In
child_will_go_down() it is checked and close_all_backend_connections()
is called only when the variable is set to true.

Problem reported and analyzed by: Emond Papegaaij
Backpatch-through: v4.2
Discussion: https://www.pgpool.net/pipermail/pgpool-general/2024-November/001938.html

src/protocol/child.c

@@ -140,6 +140,11 @@ struct timeval startTime;
 bool		stop_now = false;
 #endif
 
+/*
+ * If true, connection pool has been initialized.
+ */
+static bool connection_pool_initialized = false;
+
 /*
 * child main loop
 */
@@ -223,6 +228,7 @@ do_child(int *fds)
 	{
 		child_exit(POOL_EXIT_AND_RESTART);
 	}
+	connection_pool_initialized = true;
 
 	/*
 	 * Open pool_passwd in child process.  This is necessary to avoid the file
@@ -1308,7 +1314,7 @@ child_will_go_down(int code, Datum arg)
 	}
 
 	/* let backend know now we are exiting */
-	if (pool_connection_pool)
+	if (connection_pool_initialized)
 		close_all_backend_connections();
 }
 void