ieee802154: 6lowpan: add generic lowpan header check

This patch introduce an earlier check if a 6LoWPAN frame can be valid.
This contains at first for checking if the header contains a dispatch
byte and isn't the nalp dispatch value, which means it isn't a 6LoWPAN
packet. Also we add a check if we can derference the dispatch value by
checking if skb->len is unequal zero.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>