pgsql: Fix and simplify some code related to cryptohashes

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/15b824da97afb45f47e51b6b5b7e5eca09e5d03d

Modified Files
--------------
contrib/pgcrypto/internal.c          |  9 ++++--
src/common/checksum_helper.c         | 59 +++++++++++++++---------------------
src/include/common/checksum_helper.h |  5 +--
3 files changed, 32 insertions(+), 41 deletions(-)