pgsql: Ensure xmlFree(NULL) is a no-op instead of a core dump.

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
xml.c (r1.90 -> r1.91)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.90&r2=1.91)