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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>