tests: check for NULL before g_object_unref()

delayed_close_free() calls g_object_unref() on a variable that is
expected to possibly contain NULL (as indicated by the fact that the
NULL case is handled in my_slow_close_output_stream_close_async()).

This is dead code right now (due to a bug in GDBus), which is why it
isn't actually causing a failure.  It should still be fixed, however.

https://bugzilla.gnome.org/show_bug.cgi?id=743990

gio/tests/gdbus-close-pending.c

@@ -174,6 +174,7 @@ delayed_close_free (gpointer data)
   DelayedClose *df = data;
 
   g_object_unref (df->stream);
+  if (df->cancellable)
     g_object_unref (df->cancellable);
   g_free (df);
 }