serial: sandbox: Fix buffer underflow in puts

Fix the buffer underflow that would occur if puts is called with length
of zero.

Fixes: efa51f2bd6 ("serial: sandbox: Implement puts")
Cc: Sean Anderson <sean.anderson@seco.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>

drivers/serial/sandbox.c

@@ -114,7 +114,7 @@ static ssize_t sandbox_serial_puts(struct udevice *dev, const char *s,
 	struct sandbox_serial_priv *priv = dev_get_priv(dev);
 	ssize_t ret;
 
-	if (s[len - 1] == '\n')
+	if (len && s[len - 1] == '\n')
 		priv->start_of_line = true;
 
 	if (sandbox_serial_enabled) {