gpiolib: fix invalid pointer access in debugfs

In the Linux kernel, the following vulnerability has been resolved: gpiolib: fix invalid pointer access in debugfs If the memory allocation in gpiolib_seq_start() fails, the s->private field remains uninitialized and is later dereferenced without checking in gpiolib_seq_stop(). Initialize s->private to NULL before calling kzalloc() and check it before dereferencing it.