The original fix was done by Sergey Glukhov on 2009-02-04
All comments are copied from the original fix:


Bug#42495 updatexml: Assertion failed: xpath->context,
                     file .\item_xmlfunc.cc, line 2507
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  ((for bug#42495))
mysql-test/t/xml.test:
  test case  ((for bug#42495))
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.