V4L/DVB (5472): Isl6421: don't reference freed memory

After freeing a block there should be no reference to this block.
Signed-off-by: Thomas Viehweger <Thomas.Viehweger@marconi.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/dvb/frontends/isl6421.c

@@ -122,6 +122,7 @@ struct dvb_frontend *isl6421_attach(struct dvb_frontend *fe, struct i2c_adapter
 	/* detect if it is present or not */
 	if (isl6421_set_voltage(fe, SEC_VOLTAGE_OFF)) {
 		kfree(isl6421);
+		fe->sec_priv = NULL;
 		return NULL;
 	}