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.


mysql-test/r/xml.result:
  test result
mysql-test/t/xml.test:
  test case
sql/item_xmlfunc.cc:
  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.