Revert "Fix mismatch between first and next in drain"

This reverts commit a51b6ce4.

We can't increment first when no data is there, otherwise we might do it while the first frame was not yet decoded, messing up ordering: imagine having a framedelay of 8, and a file with 7 frames. We feed 7 frames over 8 slots, now next points to [7] (empty entry), and we start draining cause EOF. We do need next to be incremented to reach the first frame ([0]), so it can be outputted, and only then first too.

Fixes #418 (closed).