Fix memory leak when unloading XML documentation

Fix memory leak when unloading XML documentation

This patch is a modified version of a patch originally committed for the
Asterisk 11 branch in r375756.  A portion of that patch, that fixed the
memory leak during unloading XML documentation, applies to branches 1.8
and 10 as well.

The patch for this issue was modified for these two branches.

(issue ASTERISK-20648)
Reported by: Corey Farrell
Tested by: mjordan
patches:
  xmldoc-memory_leak.patch uploaded by Corey Farrell (license 5909)
........

Merged revisions 375758 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@375759 65c4cc65-6c06-0410-ace0-fbb531ad65f3