common: Fix criticals when user language getting fails

Handle the error case when the call to the accountservice dbus proxy
object fails.

https://bugzilla.gnome.org/show_bug.cgi?id=699378

panels/common/cc-common-language.c

@@ -505,7 +505,12 @@ get_lang_for_user_object_path (const char *path)
 		g_error_free (error);
 		return NULL;
 	}
+
 	props = g_dbus_proxy_get_cached_property (user, "Language");
+	if (props == NULL) {
+		g_object_unref (user);
+		return NULL;
+	}
 	lang = g_variant_dup_string (props, NULL);
 
 	g_variant_unref (props);