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