Revert a change that broke compilation

Revert a change that broke compilation

1) There is no such function as ast_ref()
2) The patch was originally credited as the one uploaded by Guenther
   Kelleter (license 6372) via issue AST-921, but the patch committed
   was not the patch referenced on the issue.
3) Guenther Kelleter's patch was actually correct. It moved the
   ast_free above the presencechange_cleanup label. I am not
   committing his change as it is not technically necesary--calling
   ast_free(NULL) is perfectly safe and I worry that moving the
   ast_free outside of the label could lead to future bugs if
   someone ever adds another failure conditional and expects
   'goto presencechange_cleanup;' to clean up after everything.

git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/1.8.11@370472 65c4cc65-6c06-0410-ace0-fbb531ad65f3