Fix crash in tls13_enc

If s->s3->tmp.new_cipher is NULL then a crash can occur. This can happen
if an alert gets sent after version negotiation (i.e. we have selected
TLSv1.3 and ended up in tls13_enc), but before a ciphersuite has been
selected.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2575)