Tighten up client status_request processing

Instead of making a positive comparison against the invalid value
that our server would send, make a negative check against the only
value that is not an error.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2953)