Return errors even if the cookie validation has succeeded

In the DTLS ClientHello processing the return value is stored in |ret| which
by default is -1. We wish to return 1 on success or 2 on success *and* we
have validated the DTLS cookie. Previously on successful validation of the
cookie we were setting |ret| to 2. Unfortunately if we later encounter an
error then we can end up returning a successful (positive) return code from
the function because we already set |ret| to a positive value.

This does not appear to have a security consequence because the handshake
just fails at a later point.

Reviewed-by: Andy Polyakov <appro@openssl.org>