gpiolib: Fix potential NULL pointer dereference in gpiod_get_label()

In the Linux kernel, the following vulnerability has been resolved:

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.