ALSA: control_led - fix initialization in the mode show callback

The str variable should be always initialized before use even if
the switch covers all cases. This is a minimalistic fix: Assign NULL,
the sprintf() may print '(null)' if something is corrupted.
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20210614071710.1786866-1-perex@perex.czSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/control_led.c

@@ -397,7 +397,7 @@ static ssize_t mode_show(struct device *dev,
 			 struct device_attribute *attr, char *buf)
 {
 	struct snd_ctl_led *led = container_of(dev, struct snd_ctl_led, dev);
-	const char *str;
+	const char *str = NULL;
 
 	switch (led->mode) {
 	case MODE_FOLLOW_MUTE:	str = "follow-mute"; break;