net/mlx5: fix memory region cache lookup

net/mlx5: fix memory region cache lookup

The Memory Region (MR) cache contains pointers to mlx5_mr.
The MR cache indexes are filled when a new MR is created. As it is
possible for MR to be created on the flight, an extra validation must be
added to avoid segmentation fault.

Fixes: b0b093845793 ("net/mlx5: use buffer address for LKEY search")
Cc: stable@dpdk.org
Signed-off-by: Xueming Li <xuemingl@mellanox.com>
Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>