Fix crash with res_fax when MALLOC_DEBUG and "core stop gracefully" are used.

Fix crash with res_fax when MALLOC_DEBUG and "core stop gracefully" are used.

Asterisk crashes if MALLOC_DEBUG is enabled when res_fax tries to
unregister its logger level.

* Make ast_logger_unregister_level() use ast_free() instead of free().
When MALLOC_DEBUG is enabled, ast_free() does not degenerate into a call
to free().  Therefore, if you allocated memory with a form of ast_malloc
you must free it with ast_free.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@334953 65c4cc65-6c06-0410-ace0-fbb531ad65f3