[MediaStream][GStreamer] Hardware decoder/sinks do not have src pads

https://bugs.webkit.org/show_bug.cgi?id=276170

Reviewed by Philippe Normand.

In the MediaStream use case, we are trying to get decoder stats from a pad probe at the src pad of the video decoder.
The problem comes with the decoder/sink elements that don't have one because they are the end of the pipeline.

* Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp:
(WebCore::MediaPlayerPrivateGStreamer::configureVideoDecoder):

Canonical link: https://commits.webkit.org/280622@main

Author: calvaris

Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp

@@ -3192,6 +3192,10 @@ void MediaPlayerPrivateGStreamer::configureVideoDecoder(GstElement* decoder)
         g_object_set(decoder, "max-errors", -1, nullptr);
 
     auto pad = adoptGRef(gst_element_get_static_pad(decoder, "src"));
+    if (!pad) {
+        GST_INFO_OBJECT(pipeline(), "the decoder %s does not have a src pad, probably because it's a hardware decoder sink, can't get decoder stats", name.get());
+        return;
+    }
     gst_pad_add_probe(pad.get(), static_cast<GstPadProbeType>(GST_PAD_PROBE_TYPE_QUERY_DOWNSTREAM | GST_PAD_PROBE_TYPE_BUFFER), [](GstPad*, GstPadProbeInfo* info, gpointer userData) -> GstPadProbeReturn {
         auto* player = static_cast<MediaPlayerPrivateGStreamer*>(userData);
         if (GST_PAD_PROBE_INFO_TYPE(info) & GST_PAD_PROBE_TYPE_BUFFER) {