Lock iax2 pvt structure when passing off to the AMI function, and make sure it...

Lock iax2 pvt structure when passing off to the AMI function, and make sure it exists. (issue #9674 reported by arabe)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@63182 65c4cc65-6c06-0410-ace0-fbb531ad65f3