Fix client verify mode to check SSL_VERIFY_PEER

The original check for != SSL_VERIFY_NONE can give surprising results
when flags SSL_VERIFY_PEER is not set, but other flags are.  Note
that SSL_VERIFY_NONE (0) is not a flag bit, it is rather the absense
of all other flag bits.

Signed-off-by: Rob Percival <robpercival@google.com>
Reviewed-by: Emilia Käsper <emilia@openssl.org>