net: cavium: fix NULL pointer dereference in cavium_ptp_put

Prevent a kernel panic on reboot if ptp_clock is NULL by checking
the ptp pointer before using it.

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Fixes: 8c56df372b ("net: add support for Cavium PTP coprocessor")
Cc: Radoslaw Biernacki <rad@semihalf.com>
Cc: Aleksey Makarov <aleksey.makarov@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cavium/common/cavium_ptp.c

@@ -75,6 +75,8 @@ EXPORT_SYMBOL(cavium_ptp_get);
 
 void cavium_ptp_put(struct cavium_ptp *ptp)
 {
+	if (!ptp)
+		return;
 	pci_dev_put(ptp->pdev);
 }
 EXPORT_SYMBOL(cavium_ptp_put);