gpio: shared: fix a NULL-pointer dereference

gpio: shared: fix a NULL-pointer dereference

The fact that CONFIG_OF is enabled does not mean that the device tree is
populated and that of_root points to a valid device node. Check if it's
NULL before trying to traverse the tree.

Fixes: a060b8c511ab ("gpiolib: implement low-level, shared GPIO support")
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/all/dbe20642-9662-40af-a593-c1263baea73b@sirena.org.uk/
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20251118200459.13969-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>