Problem: there's no guarantee that the user variable item's result_field
is assigned when we're adjusting its table read map.
  
Fix: check the result_field before using it.


mysql-test/r/user_var.result:
  Fix for bug #32260: User variables in query cause server crash
    - test result.
mysql-test/t/user_var.test:
  Fix for bug #32260: User variables in query cause server crash
    - test case.
sql/item_func.cc:
  Fix for bug #32260: User variables in query cause server crash
    - using the result_field ensure it is set.