In the OP_Column opcode, when extracting a field that is past the end of

a short record (because the row was originally inserted prior to ALTER TABLE
ADD COLUMN) then make sure the output register is fully NULL and does not
contain leftover flags (such as MEM_Ephem) from its previous use.
Fix for ticket [43107840f1c02].

FossilOrigin-Name: 24780f8ddc1683fc62180e6961dc6bfe1168f4df