appsrc: Correctly check if this is the first buffer that was queued

By checking the queue length this would also count events and caps. We
already keep track of the number of buffers separately so just use that.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1197>

gst-libs/gst/app/gstappsrc.c

@@ -1501,7 +1501,7 @@ gst_app_src_update_queued_push (GstAppSrc * appsrc, GstMiniObject * item)
      * here. This is especially useful if this was the first buffer because
      * otherwise we would have to wait until it is actually unqueued to know
      * the queued duration */
-    if (gst_queue_array_get_length (priv->queue) == 1) {
+    if (priv->queued_buffers == 1) {
       if (priv->last_segment.stop != -1
           && start_buffer_ts > priv->last_segment.stop)
         start_buffer_ts = priv->last_segment.stop;