net/mlx5: fix missing validation of null pointer

net/mlx5: fix missing validation of null pointer

Function mlx5_rxq_ibv_release() is called in several places.
Before each call except one, the input parameter is validated to make
sure it is not null.

This patch adds the validation where it is missing.
It also changes a priv_ prefix, left in a comment, to mlx5_ prefix.

Fixes: af4f09f28294 ("net/mlx5: prefix all functions with mlx5")
Cc: stable@dpdk.org
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>