I had an assert that fired in this case, dating back to when I hadn't plumbed
Result returns through much of .mp4 construction. Now I have, so there's no
excuse in having an assert here. Change to an error return, and tweak it to
not fire in the zero-duration case.
Also fix a problem in the test harness; I hadn't finished converting it for
multi-recording tests, and it was returning the wrong recording.
Because of that, I seem to have stumbled across a related problem in which
asking for zero duration of a non-zero duration recording will return a
recording::Segment with no frames, which will cause panics because its
corresponding .mp4 slices are zero-length. I just adjusted the panic message
here; I'll follow up with changes to address that.
