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`.