ixgbe: fix possible NULL pointer deference in shutdown path

After freeing the rings we were not zeroing out the ring count values.
This patch now clears these counts correctly.

Reported-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ixgbe/ixgbe_main.c

@@ -4771,6 +4771,9 @@ void ixgbe_clear_interrupt_scheme(struct ixgbe_adapter *adapter)
 		adapter->rx_ring[i] = NULL;
 	}
 
+	adapter->num_tx_queues = 0;
+	adapter->num_rx_queues = 0;
+
 	ixgbe_free_q_vectors(adapter);
 	ixgbe_reset_interrupt_capability(adapter);
 }