net/mlx5: fix destroying empty matchers list

net/mlx5: fix destroying empty matchers list

The table remove callback function is trying to destroy the
matchers list associated with table entries without checking
if the list is valid, which causes null pointer dereference.
Fixed by validating the matchers list before destroying it.

Issue can be reproduced with testpmd on Windows, when you run:
port close all

Fixes: 1872635570be ("net/mlx5: make matcher list thread safe")
Cc: stable@dpdk.org
Signed-off-by: Adham Masarwah <adham@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: Tal Shnaiderman <talshn@nvidia.com>
Tested-by: Idan Hackmon <idanhac@nvidia.com>