Fix string handling in get_combo_box_entry_text

Linus' code dropped the const qualifier from the start rating. While
fixing this I stared some more at get_combo_box_entry_text and realized
that the existing code could potentially change the "old" pointer and then
pass it to free(). Tsk-tsk-tsk.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>