MALLOC_DEBUG: Change fence magic number to be completely different from the freed magic number.

Race conditions between freeing a nul terminated string and
ast_strdup()'ing it are more likely to be detected if the fence and freed
magic numbers are completely different.
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Merged revisions 398703 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 398721 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 398726 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@398732 65c4cc65-6c06-0410-ace0-fbb531ad65f3