We should only unsubscribe to the device state event

subscription if we have previously subscribed. Otherwise
a segfault will occur.

(closes issue #13476)
Reported by: jonnt
Patches:
      13476.patch uploaded by putnopvut (license 60)
Tested by: jonnt



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@143609 65c4cc65-6c06-0410-ace0-fbb531ad65f3