shell_dbus_acquire_name: Don't assume error is set

In rare cases (mostly when the bus connection is going away),
g_dbus_proxy_call_sync can return NULL without setting an error.
Don't crash in this case.

src/main.c

@@ -68,7 +68,7 @@ shell_dbus_acquire_name (GDBusProxy  *bus,
                                                        NULL, /* cancellable */
                                                        &error)))
     {
-      g_printerr ("failed to acquire %s: %s\n", name, error->message);
+      g_printerr ("failed to acquire %s: %s\n", name, error ? error->message : "");
       if (!fatal)
         return;
       exit (1);