Fix incorrect usages of ast_realloc().

Fix incorrect usages of ast_realloc().

There are several locations in the code base where this is done:
buf = ast_realloc(buf, new_size);

This is going to leak the original buf contents if the realloc fails.

Review: https://reviewboard.asterisk.org/r/2832/
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Merged revisions 398757 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 398758 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 398759 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@398760 65c4cc65-6c06-0410-ace0-fbb531ad65f3