pgsql: Fix xslt_process() to ensure that it inserts a NULL terminator

Log Message:
-----------
Fix xslt_process() to ensure that it inserts a NULL terminator after the
last pair of parameter name/value strings, even when there are MAXPARAMS
of them. Aboriginal bug in contrib/xml2, noted while studying bug #4912
(though I'm not sure whether there's something else involved in that
report).

This might be thought a security issue, since it's a potential backend
crash; but considering that untrustworthy users shouldn't be allowed
to get their hands on xslt_process() anyway, it's probably not worth
getting excited about.

Modified Files:
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pgsql/contrib/xml2:
xslt_proc.c (r1.15 -> r1.16)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xslt_proc.c?r1=1.15&r2=1.16)