Problem:
   RelativeLocationPath can appear only after a node-set expression
   in the third and the fourth branches of this rule:
     PathExpr :: =  LocationPath
                    | FilterExpr
                    | FilterExpr '/' RelativeLocationPath
                    | FilterExpr '//' RelativeLocationPath
   XPatch code didn't check the type of FilterExpr and crashed.
Fix:
   If FilterExpr is a scalar expression
   (variable reference, literal, number, scalar function call)
   return error.