IS NULL was not checking the correct row in a HAVING context.
At the first row of a new group (where the HAVING clause is evaluated)
the column and SELECT list references in the HAVING clause should 
refer to the last row of the previous group and not to the current one. 
This was not done for IS NULL, because it was using Item::is_null() doesn't
have a  Item_is_null_result() counterpart to access the data from the 
last row of the previous group. Note that all the Item::val_xxx() functions 
(e.g. Item::val_int()) have their _result counterparts (e.g. Item::val_int_result()).

Fixed by implementing a is_null_result() (similarly to int_result()) and
calling this instead of is_null() column and SELECT list references inside
the HAVING clause.