Fix issues with skinny sessions

Fixes a couple of issues with the way skinny handles sessions by ensuring
sessions aren't used after being freed. Some other minor changes.

Review: https://reviewboard.asterisk.org/r/2272/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@379582 65c4cc65-6c06-0410-ace0-fbb531ad65f3