gst: handle clock reset

The clock time is reset to 0 after a new format is negotiated. Make sure
the time reported by the gstreamer clock is increasing in this case.

src/gst/gstpipewiresink.c

@@ -502,6 +502,9 @@ on_format_changed (void *data, const struct spa_pod *format)
 {
   GstPipeWireSink *pwsink = data;
 
+  if (format == NULL)
+	  return;
+
   if (gst_buffer_pool_is_active (GST_BUFFER_POOL_CAST (pwsink->pool)))
     pool_activated (pwsink->pool, pwsink);
 }