Fix incorrect optimization in find_builtin_icon().

When we found an icon with exactly the requested size, we'd stop
searching immediately (good), but we'd neglect to set the returned
min_difference to 0 (bad).  This caused theme_lookup_icon() to
prefer other, potentially much worse, matches over the exact one.

gtk/gtkicontheme.c

@@ -5399,6 +5399,7 @@ find_builtin_icon (const gchar *icon_name,
       
       if (difference == 0)
         {
+          min_difference = 0;
           min_icon = default_icon;
           break;
         }