Ensure xmlFree(NULL) is a no-op instead of a core dump. Per report from
commitd2a70156bb6da7df030e989a97f5613826e546ea
authortgl <tgl>
Wed, 10 Jun 2009 03:44:35 +0000 (10 03:44 +0000)
committertgl <tgl>
Wed, 10 Jun 2009 03:44:35 +0000 (10 03:44 +0000)
tree8119d8b7387450ccaf09716436f64c04c8c22cbe
parente77b167b0ac8925913f8dd2476ec6ce8ff77e72b
Ensure xmlFree(NULL) is a no-op instead of a core dump.  Per report from
Sergey Burladyan, there are at least some dank corners of libxml2 that
assume this behavior, even though their published documentation suggests
they shouldn't.

This is only really a live problem in 8.3, but the code is still there
for possible debugging use in HEAD, so patch both branches.
src/backend/utils/adt/xml.c