This can't happen because we don't implement RSA_PSK, but we probably should
check here.

Probably |sess_cert| shouldn't be attached to SSL_SESSION anyway; it's only
relevant when initializing the session and if it's accessed afterwards, it'll
be shared and cause problems.

Change-Id: Id868e523195f33c22e057f9b89dc02fe68e9b554
Reviewed-on: https://boringssl-review.googlesource.com/1153
Reviewed-by: Adam Langley <agl@google.com>