chan_agent: Fix agent_indicate() locking.

Avoid deadlock potential with local channels and simplify the locking.
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Merged revisions 378427 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@378428 f38db490-d61c-443f-a65b-d21fe96a405b