res_pjsip/pjsip_cli: fix compilation error caused by passing ast_free

When wanting to pass *free as a function pointer, ast_free_ptr has to be used
instead of ast_free. This allows it to be compiled with MALLOC_DEBUG enabled.
........

Merged revisions 404531 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@404532 f38db490-d61c-443f-a65b-d21fe96a405b