gpiolib: Fix potential NULL pointer dereference in gpiod_get_label()

In `gpiod_get_label()`, it is possible that `srcu_dereference_check()` may
return a NULL pointer, leading to a scenario where `label->str` is accessed
without verifying if `label` itself is NULL.

This patch adds a proper NULL check for `label` before accessing
`label->str`. The check for `label->str != NULL` is removed because
`label->str` can never be NULL if `label` is not NULL.

This fixes the issue where the label name was being printed as `(efault)`
when dumping the sysfs GPIO file when `label == NULL`.

Fixes: 5a646e03 ("gpiolib: Return label, if set, for IRQ only line")
Fixes: a86d2769 ("gpiolib: fix the speed of descriptor label setting with SRCU")
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20241003131351.472015-1-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>