Do not try to remove a registration scheduled item if the scheduler context...

Do not try to remove a registration scheduled item if the scheduler context has already been destroyed.

(closes issue #14580)
Reported by: alecdavis


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@179323 65c4cc65-6c06-0410-ace0-fbb531ad65f3