Fix MALLOC_DEBUG version of ast_strndup().

(closes issue ASTERISK-20349)
Reported by: Brent Eagles
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Merged revisions 372655 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 372656 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@372657 65c4cc65-6c06-0410-ace0-fbb531ad65f3