ptp: fix NULL pointer dereference in ptp_clock_register

Fix NULL pointer dereference in ptp_clock_register. The argument
"parent" of ptp_clock_register may be NULL pointer.

Fixes: 73f37068 ("ptp: support ptp physical/virtual clocks conversion")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/ptp/ptp_clock.c

@@ -236,7 +236,7 @@ struct ptp_clock *ptp_clock_register(struct ptp_clock_info *info,
 	}
 
 	/* PTP virtual clock is being registered under physical clock */
-	if (parent->class && parent->class->name &&
+	if (parent && parent->class && parent->class->name &&
 	    strcmp(parent->class->name, "ptp") == 0)
 		ptp->is_virtual_clock = true;