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: e0454311 "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

@@ -409,6 +409,7 @@ void tick_broadcast_control(enum tick_broadcast_mode mode)
 		break;
 	}
 
+	if (bc) {
 		if (cpumask_empty(tick_broadcast_mask)) {
 			if (!bc_stopped)
 				clockevents_shutdown(bc);
@@ -418,6 +419,7 @@ void tick_broadcast_control(enum tick_broadcast_mode mode)
 			else
 				tick_broadcast_setup_oneshot(bc);
 		}
+	}
 	raw_spin_unlock(&tick_broadcast_lock);
 }
 EXPORT_SYMBOL_GPL(tick_broadcast_control);