serial: mctrl_gpio: Check for NULL pointer

When using mctrl_gpio_to_gpiod, it dereferences gpios into a single
requested GPIO.  This dereferencing can break if gpios is NULL,
so this patch adds a NULL check before dereferencing it.  If
gpios is NULL, this function will also return NULL.
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Link: https://lore.kernel.org/r/20191006163314.23191-1-aford173@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_mctrl_gpio.c

@@ -66,6 +66,9 @@ EXPORT_SYMBOL_GPL(mctrl_gpio_set);
 struct gpio_desc *mctrl_gpio_to_gpiod(struct mctrl_gpios *gpios,
 				      enum mctrl_gpio_idx gidx)
 {
+	if (gpios == NULL)
+		return NULL;
+
 	return gpios->gpio[gidx];
 }
 EXPORT_SYMBOL_GPL(mctrl_gpio_to_gpiod);