gpiolib: do not allow to insert an empty gpiochip

commit 5ed41cc4 upstream.

We need to check if number of gpio is positive if there is no
such check in devicetree or acpi or whatever called before
gpiochip_add.

I suppose that devicetree and acpi do not allow insert gpiochip
with zero number but I do not know if it is enough to ignore
this check in gpiochip_add.
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[ kamal: 4.2-stable prereq ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/gpio/gpiolib.c

@@ -247,6 +247,11 @@ int gpiochip_add(struct gpio_chip *chip)
 	if (!descs)
 		return -ENOMEM;
 
+	if (chip->ngpio == 0) {
+		chip_err(chip, "tried to insert a GPIO chip with zero lines\n");
+		return -EINVAL;
+	}
+
 	spin_lock_irqsave(&gpio_lock, flags);
 
 	if (base < 0) {