Fixes crash in audiohook_write_list

Fixes crash in audiohook_write_list

The middle_frame in the audiohook_write_list function was
being freed if a audiohook manipulator returned a failure.
This is incorrect logic.  This patch resolves this and
adds detailed descriptions of how this function should work
and why manipulator failures must be ignored.

(closes issue #17052)
Reported by: dvossel
Tested by: dvossel

(closes issue #16196)
Reported by: atis

Review: https://reviewboard.asterisk.org/r/623/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@260049 65c4cc65-6c06-0410-ace0-fbb531ad65f3