Skinny: fix up possible double unlock of chan.

Return before chan is possibly unlocked a second time when hanging up
a channel in SUBSTATE_OFFHOOK.
........

Merged revisions 406987 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@406988 65c4cc65-6c06-0410-ace0-fbb531ad65f3