memory leak in xml2 contrib module

Hi,

one of my colleagues found ugly memory leak in the xml2 contrib module.
Try this:

select xpath_number('<data>' || generate_series || '</data>','/data') from
generate_series(1,500000);

This will allocate arbitrary amount of memory, and it won't release it
until the backend exits (user disconnects). We've found this in 8.4 and
I've verified it's still present in 9.0.1.

I know the xml2 module is deprecated and most of the functions was moved
into core (and behave correctly) but xslt_process function was not and
that's why xml2 module is used.

Pavel Stehule already prepared a fix (see the attachment).

regards
Tomas