- dynamically allocate the ast_jb structure that is on the channel structure

  so that channels not using a jitterbuffer don't waste as much memory
- ensure that the channel drivers that use jitterbuffers can handle a failure
  from configuring a jitterbuffer on a new channel because of a memory
  allocation error
- On passing through these channel drivers, configure the jitterbuffer before
  starting the PBX thread instead of afterwards. If the pbx fails to start for
  whatever reason, this would have caused a crash.
- Also on passing, move the increase of the usecount to after all of the
  possible failure conditions in the function
- fix a place where ast_update_use_count() was not called
- ensure that the owner channel pointer of the channel pvt strcutures is set to
  NULL in failure conditions


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@35553 65c4cc65-6c06-0410-ace0-fbb531ad65f3