avoid allocating a new win struct if possible

compton.c

@@ -1573,16 +1573,15 @@ set_opacity(Display *dpy, win *w, unsigned long opacity) {
 
 static void
 add_win(Display *dpy, Window id, Window prev, Bool override_redirect) {
+  if (find_win(dpy, id)) {
+    return;
+  }
+
   win *new = malloc(sizeof(win));
   win **p;
 
   if (!new) return;
 
-  if (find_win(dpy, id)) {
-    free(new);
-    return;
-  }
-
   if (prev) {
     for (p = &list; *p; p = &(*p)->next) {
       if ((*p)->id == prev && !(*p)->destroyed)