Work around an apparent bug in GtkFileChooserButton.

It looks like some versions of GTK+ intialize GtkFileChooserButton with a
location set to NULL and crash when you select an item from the dropdown
menu. Make sure we have a valid location set in that case.

Change-Id: If2556ef939b95d8af12581a7116a48fb46586f57
Bug: 10434
Reviewed-on: https://code.wireshark.org/review/3977
Reviewed-by: Gerald Combs <gerald@wireshark.org>