gpio: max732x: Fix possible NULL pointer dereference on i2c_new_dummy error

In max732x_probe() driver allocates dummy I2C device (if number of ports
is greater than 8) with i2c_new_dummy() but it does not check the return
value of this call.

In case of error (i2c_new_device(): memory allocation failure or I2C
address cannot be used) this function returns NULL which is later
dereferenced by i2c_smbus_read_byte() (called from max732x_readb()).
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-max732x.c

@@ -622,6 +622,13 @@ static int max732x_probe(struct i2c_client *client,
 		goto out_failed;
 	}
 
+	if (nr_port > 8 && !chip->client_dummy) {
+		dev_err(&client->dev,
+			"Failed to allocate second group I2C device\n");
+		ret = -ENODEV;
+		goto out_failed;
+	}
+
 	mutex_init(&chip->lock);
 
 	max732x_readb(chip, is_group_a(chip, 0), &chip->reg_out[0]);