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: http://svn.asterisk.org/svn/asterisk/trunk@394836 f38db490-d61c-443f-a65b-d21fe96a405b