MALFORMED XPATH EXP
Problem:
A malformed XPATH expression in the ExtractValue query is causing
a server crash. This malformed XPATH expression is resulted when 
the position attribute in the substring function contains ".." in
the beginning.
Solution:
The original crash is happening because the "../" is being evaluated
prematurely. It tries to access XML while it hasn't been parsed yet.
The premature evaluation is happening because the val_nodeset function
is being set to constant, in which case we proceed to evaluate them in
JOIN:prepare stage only. The solution to this is setting the val_nodeset
functions as non-constant. This forces us to evaluate the function in
the JOIN:exec stage and thus avoid any premature evaluation of the 
XML strings.