pinctrl: single: fix potential NULL dereference in pcs_get_function()

pinmux_generic_get_function() can return NULL and the pointer 'function'
was dereferenced without checking against NULL. Add checking of pointer
'function' in pcs_get_function().

Found by code review.

Cc: stable@vger.kernel.org
Fixes: 571aec4 ("pinctrl: single: Use generic pinmux helpers for managing functions")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Link: https://lore.kernel.org/20240808041355.2766009-1-make24@iscas.ac.cn
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Author: Ma Ke

drivers/pinctrl/pinctrl-single.c

@@ -345,6 +345,8 @@ static int pcs_get_function(struct pinctrl_dev *pctldev, unsigned pin,
 		return -ENOTSUPP;
 	fselector = setting->func;
 	function = pinmux_generic_get_function(pctldev, fselector);
+	if (!function)
+		return -EINVAL;
 	*func = function->data;
 	if (!(*func)) {
 		dev_err(pcs->dev, "%s could not find function%i\n",