v0.7.9

Never generate an endless picture, even when the length is unknown

The previous commit bounded the generated picture with -t whenever the
caller knew how long the source was, and left -shortest to cover the case
where it did not. The Linux build then failed on exactly that case: a file
with no duration passed in came out at 31 seconds, because -shortest is what
that ffmpeg ignores.

A fallback that does not hold is not a fallback. So there is no longer a
path that generates an endless stream: when the caller has no length, the
file is probed for one, and a file that still cannot say gets a fixed cap
rather than running forever. -t always has a value now.

The test for this asserts the audio's own length instead of merely "less
than ten seconds". The weaker assertion would have passed a file that was
wrong by four seconds, which is the kind of wrong nobody notices until they
have a folder full of them.