Keep old method in case of an unsupported protocol

When we're configured with no-ssl3 and we receive an SSL v3 Client Hello, we set
the method to NULL.  We didn't used to do that, and it breaks things.  This is a
regression introduced in 62f45cc27d07187b59551e4fad3db4e52ea73f2c.  Keep the old
method since the code is not able to deal with a NULL method at this time.

CVE-2014-3569, PR#3571

Reviewed-by: Emilia Käsper <emilia@openssl.org>