gpiolib: fix invalid pointer access in debugfs

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.

Fixes: e348544f7994 ("gpio: protect the list of GPIO devices with SRCU")
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20251103141132.53471-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>