tick/broadcast: Prevent NULL pointer dereference

Dan reported that the recent changes to the broadcast code introduced
a potential NULL dereference.

Add the proper check.

Fixes: e0454311903d "tick/broadcast: Sanity check the shutdown of the local clock_event"
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/tick-broadcast.c

@@ -409,14 +409,16 @@ void tick_broadcast_control(enum tick_broadcast_mode mode)
 		break;
 	}
 
-	if (cpumask_empty(tick_broadcast_mask)) {
-		if (!bc_stopped)
-			clockevents_shutdown(bc);
-	} else if (bc_stopped) {
-		if (tick_broadcast_device.mode == TICKDEV_MODE_PERIODIC)
-			tick_broadcast_start_periodic(bc);
-		else
-			tick_broadcast_setup_oneshot(bc);
+	if (bc) {
+		if (cpumask_empty(tick_broadcast_mask)) {
+			if (!bc_stopped)
+				clockevents_shutdown(bc);
+		} else if (bc_stopped) {
+			if (tick_broadcast_device.mode == TICKDEV_MODE_PERIODIC)
+				tick_broadcast_start_periodic(bc);
+			else
+				tick_broadcast_setup_oneshot(bc);
+		}
 	}
 	raw_spin_unlock(&tick_broadcast_lock);
 }