6lowpan: iphc: Fix an off-by-one check of array index

The bounds check of id is off-by-one and the comparison should
be >= rather >. Currently the WARN_ON_ONCE check does not stop
the out of range indexing of &ldev->ctx.table[id] so also add
a return path if the bounds are out of range.

Addresses-Coverity: ("Illegal address computation").
Fixes: 5609c185

 ("6lowpan: iphc: add support for stateful compression")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>