Memory leak fix

ast_xmldoc_printable returns an allocated block that must be freed by the
caller.  Fixed manager.c and res_agi.c to stop leaking these results.

(closes issue ASTERISK-22395)
Reported by: Corey Farrell
Patches:
     manager-leaks-12.patch uploaded by coreyfarrell (license 5909)
     res_agi-xmldoc-leaks.patch uploaded by coreyfarrell (license 5909)
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Merged revisions 398060 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 398061 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 398062 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@398063 65c4cc65-6c06-0410-ace0-fbb531ad65f3