C++ does not guarantee the order of parameter evaluation.
It was wrong to pass item->val_int() and item->null_value
at the same time to any function or constructor.
Adding a new helper class Longlong_null, and new methods
Item::to_longlong_null() and Item_func_hybrid_field_type::to_longlong_null_op(),
which make sure to properly call val_int()/int_op() and test null_value.
Reorganizing the rest of the code accordingly.