media formats: fix ref leak of peer for mwi subscription

Holding a reference to the peer during mwi subscriptions
resulted in a circular reference because the final event
message would not be sent until destruction of the peer.

Instead, pass the name of the peer to the event callback
so that it can fail gracefully after the peer has gone.

ASTERISK-23959
Review: https://reviewboard.asterisk.org/r/3754/
........

Merged revisions 418636 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@418962 65c4cc65-6c06-0410-ace0-fbb531ad65f3