Item_equal::val_int() checked for NULL-values by checking Item::null_value
*before* the respective ::store_value() and ::cmp(Item*) metods where called.

As Item::null_value is set by these metods, the value of 'null_value' 
is not valid until *after* ::store_value() or ::cmp() has
been called for the Item object.
      
Fix is to swap order of ::store_value()/::cmp() and checking of Item::null_value.
This pattern is widely used other places inside item_cmpfunc.cc .