V4L/DVB (4679): Fix for NULL pointer dereference oops during boot.

A fix for intermittent oops's during boot which occurs
in cx88_call_i2c_clients when dvb_attach is bringing up
the frontend.
Signed-off-by: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/cx88/cx88-i2c.c

@@ -145,7 +145,7 @@ void cx88_call_i2c_clients(struct cx88_core *core, unsigned int cmd, void *arg)
 	if (0 != core->i2c_rc)
 		return;
 
-	if (core->dvbdev) {
+	if ( (core->dvbdev) && (core->dvbdev->dvb.frontend) ) {
 		if (core->dvbdev->dvb.frontend->ops.i2c_gate_ctrl)
 			core->dvbdev->dvb.frontend->ops.i2c_gate_ctrl(core->dvbdev->dvb.frontend, 1);