Fix memory leak in XML documentation

When formatting documentation fields, the XML documentation parser calls
xmldoc_get_formatted.  This function allocates a string buffer at the
beginning of its routine.  Unfortunately, on certain code paths, it also
calls xmldoc_string_cleanup, which assumes that it will create the string
buffer.  The previously allocated string buffer is then leaked by the
xmldoc_string_cleanup routine.

Now: we don't do that.

(closes issue AST-932)
Reported by: Alexander Homig
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Merged revisions 371469 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 371491 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 371492 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@371493 65c4cc65-6c06-0410-ace0-fbb531ad65f3