platform/x86: think-lmi: Avoid potential read before start of the buffer

If length equals 0 then reading buf[length-1] will read before the start
of the buffer.

Avoid this by moving the length == 0 check up.

Cc: Mark Pearson <markpearson@lenovo.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210609151752.156902-2-hdegoede@redhat.com

drivers/platform/x86/think-lmi.c

@@ -443,10 +443,13 @@ static ssize_t kbdlang_store(struct kobject *kobj,
 	int length;
 
 	length = strlen(buf);
+	if (!length)
+		return -EINVAL;
+
 	if (buf[length-1] == '\n')
 		length--;
 
-	if (!length || (length >= TLMI_LANG_MAXLEN))
+	if (length >= TLMI_LANG_MAXLEN)
 		return -EINVAL;
 
 	memcpy(setting->kbdlang, buf, length);