Fix SSL_get_shared_ciphers()

The function SSL_get_shared_ciphers() is supposed to return ciphers shared
by the client and the server. However it only ever returned the client
ciphers.

Fixes #5317

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6115)