authz-list-file: Fix crash when filename is not set

authz-list-file: Fix crash when filename is not set

Fix the following crash:

  $ qemu-system-x86_64 -object authz-list-file,id=obj0
  qemu-system-x86_64: -object authz-list-file,id=obj0: GLib: g_file_get_contents: assertion 'filename != NULL' failed
  Segmentation fault (core dumped)

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-Id: <20201008202713.1416823-2-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>