6lowpan: Off by one handling ->nexthdr

NEXTHDR_MAX is 255.  What happens here is that we take a u8 value
"hdr->nexthdr" from the network and then look it up in
lowpan_nexthdr_nhcs[].  The problem is that if hdr->nexthdr is 0xff then
we read one element beyond the end of the array so the array needs to
be one element larger.

Fixes: 92aa7c65 ("6lowpan: add generic nhc layer interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Acked-by: Alexander Aring <aring@mojatatu.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>