Refactor videoroom hangup media internal.

[atoppi]

This PR addresses a crash that occurred when a videoroom subscriber session gets quickly destroyed while being established.

When this race condition happens an invalid (freed) subscriber session gets appended to the subscribers list tied to a specific publisher. Once that publisher leaves, Janus traverses the list and references again that freed session, making the process halt.

This patch basically introduces a refactoring of the hangup media methods that will avoid appending an invalid session to the publisher list.

Notice that the signature of janus_videoroom_hangup_media_internal has been changed and the method now accepts a gpointer (aka void*) in place of a janus_plugin_session*.

[lminiero]

Merged, as this was tested extensively already.