channel: Add a missing ast_channel_unlock when allocating a Surrogate channel.
........

Merged revisions 404332 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@404333 f38db490-d61c-443f-a65b-d21fe96a405b