net: ipv6: check for NET_IPV6_NEXTHDR_NONE earlier

By definition, NET_IPV6_NEXTHDR_NONE is void. So we must stop processing
before trying to read any data, since we will start reading values that
are outside the Extension Header (likely the payload, if any).

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>