xsltEvalUserParams() and xsltQuoteUserParams() are susceptible to integer...

xsltEvalUserParams() and xsltQuoteUserParams() are susceptible to integer overflow when iterating through const char** array

* libxslt/variables.c:
(xsltEvalUserParams):
(xsltQuoteUserParams):
- Prevent integer overflow by changing the type of `indx`
  variables from `int` to `size_t`.