net/mlx4: fix drop queue error handling

net/mlx4: fix drop queue error handling

The function mlx4_drop_get() creates pointer to a struct mlx4_drop and
if needed allocates by rte_malloc.

If the allocation is failed the function goes to label “error”, and
there does dereference to a null pointer.

Skip resources cleaning when the memory allocation is failed.

Coverity issue: 146206
Coverity issue: 146146
Fixes: d3a7e09234e4 ("net/mlx4: allocate drop flow resources on demand")
Cc: stable@dpdk.org
Signed-off-by: Michael Baum <michaelba@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>