libmm-glib,firmware-update-settings: handle null self pointer

mm_firmware_update_settings_get_variant() checks for a null `self'
pointer when accessing `self->priv->method', but doesn't perform the
null check when accessing other members of
MMFirmwareUpdateSettingsPrivate.

This patch fixes mm_firmware_update_settings_get_variant() to fully
handle a null `self' pointer.

libmm-glib/mm-firmware-update-settings.c

@@ -174,7 +174,7 @@ mm_firmware_update_settings_get_variant (MMFirmwareUpdateSettings *self)
     g_variant_builder_add (&builder, "u", method);
 
     g_variant_builder_open (&builder, G_VARIANT_TYPE ("a{sv}"));
-    {
+    if (self) {
         g_variant_builder_add (&builder,
                                "{sv}",
                                PROPERTY_DEVICE_IDS,