Fix ALTER TABLE DROP COLUMN so that it generates valid bytecode even when

operating on a corrupt database and using PRAGMA writable_schema=ON.
dbsqlfuzz 5f09e7bcc78b4954d06bf9f2400d7715f48d1fef

FossilOrigin-Name: b65f4f763979ee9e0b943c787609ea22d6f7e01d41dfc1e084ec50a085a3550c