Fix error message for s_server -psk option

Previously if -psk was given a bad key it would print "Not a hex
number 's_server'".

CLA: Trivial

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/8113)