Correct value of DH_CHECK_PUBKEY_INVALID

A new return value for DH_check_pub_key was recently added:
DH_CHECK_PUBKEY_INVALID. As this is a flag which can be ORed with other
return values it should have been set to the value 4 not 3.

RT#4278

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