gpiolib: Fix potential NULL pointer dereference in gpiod_get_label()

Vulnerability Description

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`.

References

• https://git.kernel.org/stable/c/9ee4b907d7a5d7a53b4ff7727c371ff3d44ccbbb
• https://git.kernel.org/stable/c/7b99b5ab885993bff010ebcd93be5e511c56e28a