fix SIGSEGV when choosing Sounds from Actions menu in Finch

Testing Done:
Opened Sounds window in Finch.
There's another location in Finch where 'gnt_combo_box_get_dropdown' is called, which I would've liked to test, but I couldn't reach it. It's 'update_selected_account' in gntrequest.c which is triggered by some signal. The signal connection is established when username and accountlist fields are requested in 'finch_request_fields', but I didn't find a window in Finch where that's done (which doesn't mean it doesn't exist, just that I'm too unfamiliar with Finch to know where to look).

Reviewed at https://reviews.imfreedom.org/r/793/