pinctrl: check the return value of pinmux_ops::get_function_name()

commit 4002ee9 upstream.

While the API contract in docs doesn't specify it explicitly, the
generic implementation of the get_function_name() callback from struct
pinmux_ops - pinmux_generic_get_function_name() - can fail and return
NULL. This is already checked in pinmux_check_ops() so add a similar
check in pinmux_func_name_to_selector() instead of passing the returned
pointer right down to strcmp() where the NULL can get dereferenced. This
is normal operation when adding new pinfunctions.

Cc: stable@vger.kernel.org
Tested-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Bartosz Golaszewski

drivers/pinctrl/pinmux.c

@@ -337,7 +337,7 @@ static int pinmux_func_name_to_selector(struct pinctrl_dev *pctldev,
 	while (selector < nfuncs) {
 		const char *fname = ops->get_function_name(pctldev, selector);
 
-		if (!strcmp(function, fname))
+		if (fname && !strcmp(function, fname))
 			return selector;
 
 		selector++;