It's possible to have a given order greater than the number of answers;
we don't have any validation rules for that. So the check for the number
of answers isn't enough.
Checking the maximum given order in the answers is safer. Another option
would be to reorder the answers every time we add a new one, but I'm not
sure whether that's the expected behaviour.
Note even after this change the action is not thread-safe, as it is
possible to create two questions with the same given order with two
simultaneous requests.
