virtual: CMD_GotoDesk: silence compiler warning

Even though "m" will be set, still explicitly set it to NULL upon
initialisation, so that GCC is kept happy.

fvwm/virtual.c

@@ -2548,7 +2548,7 @@ void CMD_DesktopSize(F_CMD_ARGS)
  */
 void CMD_GotoDesk(F_CMD_ARGS)
 {
-	struct monitor  *m, *m_loop;
+	struct monitor  *m = NULL, *m_loop;
 	char		*token;
 	int		 new_desk;