If a URL can't be loaded, we might end up with a NULL file. Handle that
case properly by creating an invalid image instead and don't crash or
complain to stderr when files are NULL.
This was broken since 0886ade182
A new reftest has been included. We need a reftest instead of a
CSS parser test, because the error only becomes visible when
compute()ing the actual image.
Fixes#4373
(cherry picked from commit ac8c4245b2)
36cb2e932c