net/mlx4_core: Fix access to uninitialized index

net/mlx4_core: Fix access to uninitialized index

commit 2bb07e155bb3e0c722c806723f737cf8020961ef upstream.

Prevent using uninitialized or negative index when handling
steering entries.

Fixes: b12d93d63c32 ('mlx4: Add support for promiscuous mode in the new steering model.')
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>