Error out immediately on empty ciphers list.

Error out immediately on empty ciphers list.

A 0-length ciphers list is never permitted. The old code only used to
reject an empty ciphers list for connections with a session ID. It
would later error out on a NULL structure, so this change just moves
the alert closer to the problem source.

Reviewed-by: Rich Salz <rsalz@openssl.org>