Fix NPN protocol name list validation

Since 50932c4a "PACKETise ServerHello processing",
ssl_next_proto_validate() incorrectly allows empty protocol name.
draft-agl-tls-nextprotoneg-04[1] says "Implementations MUST ensure that
the empty string is not included and that no byte strings are
truncated."
This patch restores the old correct behavior.

[1] https://tools.ietf.org/html/draft-agl-tls-nextprotoneg-04



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