Fix crash in gtk_fixed_remove()

b3f6f67c changed the loop from while() to for(), but the line to move to
the next child was kept, so we could go past the end of the list.

https://bugzilla.gnome.org/show_bug.cgi?id=641176

gtk/gtkfixed.c

@@ -526,8 +526,6 @@ gtk_fixed_remove (GtkContainer *container,
 
           break;
         }
-
-      children = children->next;
     }
 }