If we don't match registrar when destroying a context, it can cause a crash.

(closes issue #12835)
 Reported by: ys
 Patches: 
       pbx.c.diff uploaded by ys (license 281)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@123358 65c4cc65-6c06-0410-ace0-fbb531ad65f3