fix: crash when use no_border_when_single

src/maomao.c

@@ -1077,7 +1077,7 @@ bool check_hit_no_border(Client *c) {
     }
   }
 
-  if (no_border_when_single && c->mon->visible_clients == 1) {
+  if (no_border_when_single && c && c->mon && c->mon->visible_clients == 1) {
     hit_no_border = true;
   }
   return hit_no_border;