pgsql: Clean up some SCRAM attribute processing

Clean up some SCRAM attribute processing

Correct the comment for read_any_attr().  Give a clearer error message
when parsing at the end of the string, when the client-final-message
does not contain a "p" attribute (for some reason).

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/2fb8a15b-de35-682d-a77b-edcc9c52fa12%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/db1f28917bac5e008dcb2653a54e73d2d0571e06

Modified Files
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src/backend/libpq/auth-scram.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)