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.
........

Merged revisions 398703 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@398721 f38db490-d61c-443f-a65b-d21fe96a405b