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-1.8.patch uploaded by coreyfarrell (license 5909)
     res_agi-xmldoc-leaks.patch uploaded by coreyfarrell (license 5909)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@398060 f38db490-d61c-443f-a65b-d21fe96a405b