Adds pass-through support for codec CELT.

This patch adds pass-through support for CELT.  CELT
formats are defined in codecs.conf and can be configured
to any sample rate a CELT endpoint supports.  This patch also
addresses a crash in channel.c resulting from a frame list being
freed incorrectly.  This crash was discovered while testing a CELT
translator which had to split encoded audio into multiple frames.
The codec translator is not a part of this patch, but may be
contributed in the future.

Review: https://reviewboard.asterisk.org/r/1294/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@326855 65c4cc65-6c06-0410-ace0-fbb531ad65f3