Fix crash on syslogger startup

When syslogger starts up, ListenSockets is still NULL. Don't try to
pfree it. Oversight in commit e29c464395.

Reported-by: Michael Paquier
Discussion: https://www.postgresql.org/message-id/ZR-uNkgL7m60lWUe@paquier.xyz

src/backend/postmaster/postmaster.c

@@ -2565,10 +2565,13 @@ ClosePostmasterPorts(bool am_syslogger)
 	 * EXEC_BACKEND mode.
 	 */
 #ifndef EXEC_BACKEND
-	for (int i = 0; i < NumListenSockets; i++)
-		StreamClose(ListenSockets[i]);
+	if (ListenSockets)
+	{
+		for (int i = 0; i < NumListenSockets; i++)
+			StreamClose(ListenSockets[i]);
+		pfree(ListenSockets);
+	}
 	NumListenSockets = 0;
-	pfree(ListenSockets);
 	ListenSockets = NULL;
 #endif