user input not sanitized

Arguments passed to the "/skipcart" callback are not checked prior to passing them on to other functions, resulting in php errors.

To reproduce the error, go to a link like this: example.com/skipcart/test . "test" is passed to cart_add_item(), which in turn is passed to node_load(), which throws a php error.

A better portion of code might be to replace this:

function skipcart_item() {
  if ( !arg(1)) {
    drupal_set_message("No arg passed to skipcart.",'error');
    return 'Exiting';
  }
  $product_nid = arg(1);

  // empty the cart
  // cart.module line 1058
  // function cart_empty($cookie_id = null) {
  // this only happens if they have it turned on
  if ( variable_get('skipcart_clearcart','0')) {
    cart_empty();
    watchdog('Skipcart',"Cart emptied in preparation for skipcarting $product_nid");
  }

  // add this item
  // cart.module line 946
  // function cart_add_item($nid, $qty = NULL, $data = NULL) {
  cart_add_item($product_nid,1);
  watchdog('Skipcart',"NID $product_nid added to new cart");

with this:

function skipcart_item() {
  if ( !($product_nid = (int)arg(1))) {
    drupal_set_message(arg(1) ? "Invalid arg passed to skipcart" : "No arg passed to skipcart.",'error');
    return 'Exiting';
  }

  // empty the cart
  // cart.module line 1058
  // function cart_empty($cookie_id = null) {
  // this only happens if they have it turned on
  if ( variable_get('skipcart_clearcart','0')) {
    cart_empty();
    watchdog('Skipcart',"Cart emptied in preparation for skipcarting $product_nid");
  }

  // add this item
  // cart.module line 946
  // function cart_add_item($nid, $qty = NULL, $data = NULL) {
  cart_add_item($product_nid,1);
  watchdog('Skipcart',"NID $product_nid added to new cart");