Fixed refcounting problems with chanspy AMI support.

The ast_multi_channel_blob_get_channel function does not bump the refcount on
the channel snapshot that it returns. This is typical for Stasis message
payloads, since being immutable means that the object won't get unreffed out
from underneath you.

The manager code for chanspy was unreffing the snapshots it got out of the
multi-channel blob, which was one unref too many.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390584 65c4cc65-6c06-0410-ace0-fbb531ad65f3