Fix long-standing segfault when accept() or one of the calls made right

after accepting a connection fails, and the server is compiled with GSSAPI
support. Report and patch by Alexander V. Chernikov, bug #5731.

src/backend/postmaster/postmaster.c

@@ -1975,7 +1975,7 @@ ConnCreate(int serverFd)
 		if (port->sock >= 0)
 			StreamClose(port->sock);
 		ConnFree(port);
-		port = NULL;
+		return NULL;
 	}
 	else
 	{