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: e348544 ("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>

Author: Bartosz Golaszewski

drivers/gpio/gpiolib.c

@@ -5296,6 +5296,8 @@ static void *gpiolib_seq_start(struct seq_file *s, loff_t *pos)
 	struct gpio_device *gdev;
 	loff_t index = *pos;
 
+	s->private = NULL;
+
 	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
 	if (!priv)
 		return NULL;
@@ -5329,7 +5331,11 @@ static void *gpiolib_seq_next(struct seq_file *s, void *v, loff_t *pos)
 
 static void gpiolib_seq_stop(struct seq_file *s, void *v)
 {
-	struct gpiolib_seq_priv *priv = s->private;
+	struct gpiolib_seq_priv *priv;
+
+	priv = s->private;
+	if (!priv)
+		return;
 
 	srcu_read_unlock(&gpio_devices_srcu, priv->idx);
 	kfree(priv);