net/bnxt: avoid null pointer dereference

net/bnxt: avoid null pointer dereference

Commit "bd0a14c99f65" enables the creation of a dedicated completion
ring for asynchronous event handling instead of handling these
events on a receive completion ring on non Stingray Platforms.

This causes a segfault due to NULL pointer dereference in
bnxt_alloc_async_cp_ring() on stingray. Fix this by checking the
pointer validity before accessing it.

Fixes: bd0a14c99f65 ("net/bnxt: use dedicated CPR for async events")
Cc: stable@dpdk.org
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Rahul Gupta <rahul.gupta@broadcom.com>
Reviewed-by: Lance Richardson <lance.richardson@broadcom.com>