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

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

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/contrib/xml2:
xpath.c (r1.8 -> r1.8.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xpath.c?r1=1.8&r2=1.8.2.1)