sub clips broken on 17.12.3

[Mikko Rapeli]

Hi,

Just noticed on Debian that 17.12.3 has broken sub-clip support which used
to work in 17.12.1 and earlier. Some sub clips don't play on timeline while
they play correctly project bin. They don't play as in the preview freezes
to last frame of previous clip. Can't reproduce with generated counter clip
or simple mp4 file in a test project but on a real on several clips with
sub clips are broken like this on timeline.

-Mikko