Fix a segmentation fault when using an extension with CID matching and no CID.

Attempting to call an extension which used Caller ID matching with a channel that
has an empty caller id string would result in a segmentation fault.

(closes issue ASTERISK-18392
Reported By: Ales Zelenik
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Merged revisions 344608 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 344609 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@344610 65c4cc65-6c06-0410-ace0-fbb531ad65f3