Fix videoroom crash on janus_videoroom_hangup_media_internal

[spscream]

possible fix of #3627

[lminiero]

From a quick glance, the use of a mutex in janus_videoroom_subscriber_free is a big no. That's the callback that's invoked when the reference count gets to zero, and it's unpredictable when it will be called, especially in terms of which mutex is locked when it happens. There's a high risk of encountering random inverse order of mutex locking because of this.

[spscream]

ok, I see.
Another idea is to make increment of subscriber->ref on adding to publisher streams? So subscriber won't be freed early and crash won't occure.