owr_gst_media_source: owr_media_source_release_source_default: assertion 'media_source->priv->source_bin' failed

[pavels]

owr_gst_media_source is missing release_source method, so default is used but that is completely unsuitable, as there is lot of stuff missing owr_gst_media_source

Either it is needed to add stub release_source as there is nothing to release at this moment or this needs to be addressed https://github.com/EricssonResearch/openwebrtc/issues/382

The stub pull request is super simple, I can do it if it is wanted.

Now you get the error in the topic, when you want to use owr_gst_media_source as local source, that is not very good.