Don't crash on INFO automon request with no channel

Don't crash on INFO automon request with no channel

AST-2011-014. When automon was enabled in features.conf, it was possible
to crash Asterisk by sending an INFO request if no channel had been
created yet.

(closes issue ASTERISK-18805)
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Merged revisions 347530 from http://svn.asterisk.org/svn/asterisk/branches/1.6.2
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Merged revisions 347531 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 347532 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@347533 65c4cc65-6c06-0410-ace0-fbb531ad65f3