Commit 5d5fdb77e9 changed details of the decoding control flow, and
called it a "high-risk" change. It turns out that this broke with with
hwdec copy mode, where there is some sort of delay queue (supposedly
increases efficiency, but more likely worthless cargo-cult).
It simply used the wrong (basically inverted) condition for the draining
case.
This was the only case that did not work properly. Other tests,
including video/audio decoding errors, software decoding fallbacks,
etc., seemed to work well. Might still not be exhaustive, as there are
so many corner cases.
Also change two error code returns. This don't/shouldn't really matter,
though the second error code led it to return both a frame and
AVERROR_EOF, which is unexpected, and makes lavc_process() leak a frame.
But also see next commit.
Fixes: 5d5fdb77e9
edc6075fa3