Early add a reference to a subscriber in videoroom handler.

[atoppi]

This PR tries to fix a race condition that might occur in the videoroom plugin when a subscriber gets destroyed while the plugin thread is handling a subscriber message.

The proposed fix is to early add a reference to the subscriber in the message handler while the sessions_mutex is stil taken by the thread. This should avoid heap-use-after-free errors when trying to access a freed subscriber later in the handler code.

[lminiero]

Merging.