Fix endless loop on srp app when listing users

With the -list option the srp app loops on the main while() endlessly,
whether users were given on the command line or not. The loop should
be stopped when in list mode and there are no more users.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3489)