XPath operator idiv
Bugfix: xpath function call changing position

When calling a function that in itself parses another
xpath expression, M.position was changed.
Small safety net for messy situations

Sometimes (mostly related to incorrect XPath expressions)
I get the error that the dispatch table (layoutxml) is nil. Let's at
least give an error message and don't handle the error at
lua level.
Small formatting correction on w(...)