[Done v1.3.9d] No numeric check for quantity update on shopping_cart page

v1.3.9b, fresh install.

When the quantity boxes are shown on the shopping cart (for quantity update), there is no numeric checking for the value typed into the box. Entering a non-numeric value, like 'a', and then pressing the Update icon results in the product being deleted from the cart.

/includes/classes/shopping_cart.php
around line 1530 you'll see this section of code.
Add the new lines as shown:```
function actionUpdateProduct($goto, $parameters) {
global $messageStack;

for ($i=0, $n=sizeof($_POST['products_id']); $i<$n; $i++) {
  $adjust_max= 'false';
 [B] if (!is_numeric($_POST['cart_quantity'][$i]) || $_POST['cart_quantity'][$i] < 0) {
    $messageStack->add_session('header', ERROR_CORRECTIONS_HEADING . ERROR_PRODUCT_QUANTITY_UNITS_SHOPPING_CART . zen_get_products_name($_POST['products_id'][$i]), 'error');
    continue;
  }[/B]