pgsql: Fix core dump in contrib/xml2's xpath_table() when the input

Log Message:
-----------
Fix core dump in contrib/xml2's xpath_table() when the input query returns
a NULL value. Per bug #4058.

Tags:
----
REL8_3_STABLE

Modified Files:
--------------
pgsql/contrib/xml2:
xpath.c (r1.16 -> r1.16.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xpath.c?r1=1.16&r2=1.16.2.1)