net/bnxt: add null check for mark table

net/bnxt: add null check for mark table

During the port start, driver ignores mark table allocation failure.
This could cause a segfault due to NULL pointer dereference in
bnxt_set_mark_in_mbuf(). Fix this by checking the pointer validity
before accessing it.

Fixes: b87abb2e55cb ("net/bnxt: support marking packet")
Cc: stable@dpdk.org
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>