Also unref the pvt when we delete the provisional keepalive job.

(closes issue #16774)
 Reported by: kowalma
 Patches: 
       20100315__issue16774.diff.txt uploaded by tilghman (license 14)
 Tested by: falves11, jamicque

Review: https://reviewboard.asterisk.org/r/591/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@257191 65c4cc65-6c06-0410-ace0-fbb531ad65f3