ignore AST_CONTROL_PVT_CAUSE_CODE without any messages

(closes issue ASTERISK-23336)
Reported by: Alexander Semych
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Merged revisions 408838 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 408839 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@408840 65c4cc65-6c06-0410-ace0-fbb531ad65f3