Don't pass STUN packets through the SRTP unprotect function.

(closes issue AST-1036)
Reported by: jbigelow
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Merged revisions 378553 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 378555 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@378557 65c4cc65-6c06-0410-ace0-fbb531ad65f3