Prevent chanspy from binding to zombie channels

Prevent chanspy from binding to zombie channels

This patch addresses a bug with chanspy on local channels which roughly 50% of the time
would create a situation where chanspy can latch onto a zombie channel, keeping the zombie
alive forever and causing the channel doing the spying to never be able to hang up.

(closes issue ASTERISK-19493)
Reported by: lvl
Review: https://reviewboard.asterisk.org/r/1819/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@359892 65c4cc65-6c06-0410-ace0-fbb531ad65f3