pgsql: Get rid of another unconstify through API changes

Get rid of another unconstify through API changes

This also makes the code in read_client_first_message() more similar
to read_client_final_message().

Reported-by: Mark Dilger <hornschnorter@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/53a28052-f9f3-1808-fed9-460fd43035ab%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/86eea78694ce0d95e74cc82cc29c096d66a9accd

Modified Files
--------------
src/backend/libpq/auth-scram.c | 56 +++++++++++++++++++++---------------------
src/backend/libpq/auth.c       |  2 +-
src/include/libpq/scram.h      |  2 +-
3 files changed, 30 insertions(+), 30 deletions(-)