transcode: don't fail spu track if we don't have something to render

SPU track works even if it doesn't have something to render at given time.
Fixes soverlay and subtitles in sout.
zvbi: fix duration on text subtitle

Missing 0 from commit 18305c12b6469c82fd84560f3935389662bc0475 and change to
use CLOCK_FREQ so it's easier to understand.