endpoints: Keep a reference to channel ids when creating snapshot.

endpoints: Keep a reference to channel ids when creating snapshot.

The snapshot process for endpoints uses the channel ids present
on the endpoint itself. Without keeping a reference it was possible
for the strings to be freed underneath any consumer of an endpoint
snapshot.

A reference is now held by the snapshot to the channel ids and
released when the snapshot is destroyed.

(issue ASTERISK-22801)
Reported by: Matt Jordan
........

Merged revisions 403542 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403543 65c4cc65-6c06-0410-ace0-fbb531ad65f3