Fix xslt_process() to ensure that it inserts a NULL terminator after the

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.