* gtkutil.c: Integer overflow fixes.

* gtkutil.c: Integer overflow fixes.

(get_utf8_string, xg_store_widget_in_map):
Check for size-calculation overflow.
(get_utf8_string): Use ptrdiff_t, not size_t, where either will
do, as we prefer signed integers.
(id_to_widget.max_size, id_to_widget.used)
(xg_store_widget_in_map, xg_remove_widget_from_map)
(xg_get_widget_from_map, xg_get_scroll_id_for_window)
(xg_remove_scroll_bar, xg_update_scrollbar_pos):
Use and return ptrdiff_t, not int.
(xg_gtk_scroll_destroy): Don't assume ptrdiff_t fits in int.
* gtkutil.h: Change prototypes to match the above.