Check coupon discount is not greater than total

This improves the situation where reward points and a coupon is use on
the same order.
https://github.com/opencart/opencart/issues/937

https://github.com/opencart/opencart/commit/e0ccfc86967cb309d8618f1f5f8eccb4058891dc
Undefined index errors when validating reviews

Caused by bots trying to spam product reviews
Only write reviews when reviews are enabled
Fix for guest group tax calculation

https://github.com/opencart/opencart/issues/942