fix locking issue on empty callList

(issue ASTERISK-19298)
Reported by:
        Dmitry Melekhov
Patches:
        ASTERISK-18322-2.patch
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Merged revisions 369146 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 369147 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369148 65c4cc65-6c06-0410-ace0-fbb531ad65f3