php cart bugfix

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-germanized/trunk@1020345 b8457f37-d9ea-0310-8a92-e5e31aec5664
check if is object before calling

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-germanized/trunk@1020349 b8457f37-d9ea-0310-8a92-e5e31aec5664