GtkFileChooser: Don't crash if invisible files are deleted

GtkFileChooser: Don't crash if invisible files are deleted

This is a crash that has been around for a long time, as
can be seen here:
https://bugzilla.redhat.com/show_bug.cgi?id=1048388
https://bugzilla.redhat.com/show_bug.cgi?id=984375
https://bugzilla.redhat.com/show_bug.cgi?id=1159015
https://bugzilla.redhat.com/show_bug.cgi?id=1059187
https://bugzilla.redhat.com/show_bug.cgi?id=1122172
https://bugzilla.redhat.com/show_bug.cgi?id=1016895
https://bugzilla.redhat.com/show_bug.cgi?id=1133235
https://bugzilla.redhat.com/show_bug.cgi?id=1077500
https://bugzilla.redhat.com/show_bug.cgi?id=1054378
https://bugzilla.redhat.com/show_bug.cgi?id=1173212

Fix suggested by Benjamin Otte.