HID: hid-lg4ff: Add check for empty lbuf

If an empty buf is received, lbuf is also empty. So lbuf is
accessed by index -1.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: f31a2de3 ("HID: hid-lg4ff: Allow switching of Logitech gaming wheels between compatibility modes")
Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-lg4ff.c

@@ -872,6 +872,12 @@ static ssize_t lg4ff_alternate_modes_store(struct device *dev, struct device_att
 		return -ENOMEM;
 
 	i = strlen(lbuf);
+
+	if (i == 0) {
+		kfree(lbuf);
+		return -EINVAL;
+	}
+
 	if (lbuf[i-1] == '\n') {
 		if (i == 1) {
 			kfree(lbuf);