Don't attempt free of NULL ptr in pbx.c handle_presencechange

(closes issue AST-921)
Reported by: Guenther Kelleter
Patches:
    nullptr.patch uploaded by Guenther Kelleter (license 6372)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370466 65c4cc65-6c06-0410-ace0-fbb531ad65f3