Fix and simplify some code related to cryptohashes
This commit addresses two issues:
- In pgcrypto, MD5 computation called pg_cryptohash_{init,update,final}
without checking for the result status.
- Simplify pg_checksum_raw_context to use only one variable for all the
SHA2 options available in checksum manifests.
Reported-by: Heikki Linnakangas
Discussion: https://postgr.es/m/
f62f26bb-47a5-8411-46e5-
4350823e06a5@iki.fi