erdtlssrtpdemux: Use gst_buffer_extract() instead of mapping manually
erdtlssrtpdemux: Use gst_buffer_extract() instead of mapping manually

And also catch empty and unmappable buffers better, especially don't
just continue with arbitrary data if a buffer is unmappable.

ford-prefect/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins,ossy-szeged/openwebrtc-gst-plugins,sancane/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.80,ossy-szeged/openwebrtc-gst-plugins,ford-prefect/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.80,EricssonResearch/openwebrtc-gst-plugins,sancane/openwebrtc-gst-plugins,ossy-szeged/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.8,deepikateriar/openwebrtc-plugins-0.8,sancane/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.80,ford-prefect/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins,deepikateriar/openwebrtc-plugins-0.8,deepikateriar/openwebrtc-plugins-0.80,deepikateriar/openwebrtc-plugins-0.8,ossy-szeged/openwebrtc-gst-plugins,ford-prefect/openwebrtc-gst-plugins