wayland: Guard against nonsense sizes being configured

With the fixes from !1638 (merged), it shouldn't be possible for this to happen any more. However, non-positive sizes make no sense regardless, so if this does somehow happen, let's make sure something reasonable happens.

The practical result of this assertion being hit is that we emit a critical warning and then behave the same as if !1634 (closed) had been merged, which is known to solve the issue for the submitter.