Fix string growth algorithm for XML presence bodies.

pjpidf_print() does not return < 0 if there is not enough
room for the document to be printed. Rather, it returns
39, the length of the XML prolog.

The algorithm also had a bug in that it would return if
it attempted to grow the string larger.
........

Merged revisions 416442 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@416444 65c4cc65-6c06-0410-ace0-fbb531ad65f3