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
Ping-Bug: 10434
Reviewed-on: https://code.wireshark.org/review/3977
Reviewed-by: Gerald Combs <gerald@wireshark.org>
(cherry picked from commit 5e3a8fa85201b7f886ad41b85e6cf8c9eca4a978)
Reviewed-on: https://code.wireshark.org/review/3978
Reviewed-by: Evan Huus <eapache@gmail.com>