gkeyfile: Clear a variable after freeing it
gkeyfile: Clear a variable after freeing it

find_file_in_data_dirs() doesn’t actually clear output_path to NULL on
failure, so this prevents a use-after-free on that (fd == -1) error
path.

Spotted by Coverity (CID: #1352981).

MathieuDuponchelle/glib,johne53/MB3Glib,Distrotech/glib,johne53/MB3Glib,endlessm/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,johne53/MB3Glib,MathieuDuponchelle/glib,Distrotech/glib,MathieuDuponchelle/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,Distrotech/glib,endlessm/glib,Distrotech/glib,Distrotech/glib,johne53/MB3Glib