don't construct a "default value field" by moving
field's ptr/null_ptr. Field can have its null_ptr moved to
extra_null_bitmap for BEFORE triggers. Perhaps there can be other
reasons for null_ptr and ptr not to be at the right offset to each other.
Instead, use pointers from TABLE_SHARE::field, which always point
to default values. Except when there's no TABLE_SHARE::field,
which can happen for TEMPTABLE views, for example, but these views
are not updatable anyway.
Add an assert to Field::move_field_offset() to ensure it's only
used for appropriately set ptr/null_ptr pairs.
when an internal temporary table field is created from a real field,
a new temp field should only copy a default from the source field
when the latter has it
This can cause crashes when accessing already released memory
The issue was the Item_default created a internal field, pointing to
share->default_values, to be used with the DEFAULT() function.
This does not work for BLOB fields as these are freed at end of query.
Fixed by storing BLOB field data inside and area allocated by
Item_default_value, like we do for nondeterministic default values.
