pgsql: Add missing error handling in pg_md5_hash().

Add missing error handling in pg_md5_hash().

It failed to provide an error string as expected for the
admittedly-unlikely case of OOM in pg_cryptohash_create().
Also, make it initialize *errstr to NULL for success,
as pg_md5_binary() does.

Also add missing comments.  Readers should not have to
reverse-engineer the API spec for a publicly visible routine.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/587de223f03e6086d511dab16b17406eb21277ce

Modified Files
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src/common/md5_common.c | 13 +++++++++++++
1 file changed, 13 insertions(+)