Minor optimizations.

* Made ast_audiohook_detach_list() and ast_audiohook_write_list_empty()
NULL tolerant.

* Made ast_audiohook_detach_list() return void since it is a destructor.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394836 65c4cc65-6c06-0410-ace0-fbb531ad65f3