bnxt_en: Fix NULL pointer dereference at bnxt_free_irq().

When open fails during ethtool -L ring change, for example, the driver
may crash at bnxt_free_irq() because bp->bnapi is NULL.

If we fail to allocate all the new rings, bnxt_open_nic() will free
all the memory including bp->bnapi.  Subsequent call to bnxt_close_nic()
will try to dereference bp->bnapi in bnxt_free_irq().

Fix it by checking for !bp->bnapi in bnxt_free_irq().

Fixes: e5811b8c09df ("bnxt_en: Add IRQ remapping logic.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -6090,7 +6090,7 @@ static void bnxt_free_irq(struct bnxt *bp)
 	free_irq_cpu_rmap(bp->dev->rx_cpu_rmap);
 	bp->dev->rx_cpu_rmap = NULL;
 #endif
-	if (!bp->irq_tbl)
+	if (!bp->irq_tbl || !bp->bnapi)
 		return;
 
 	for (i = 0; i < bp->cp_nr_rings; i++) {