hacky fix for missing segment near the end of dst file

[Hark0nnen]

sushi doesnt not handle missing segments in destination file vert well (it ouputs corresponding subtitles with a pretty random timings), but there is a particularly bad case when missing segment is near the end of dst fiel and is longer than matching segment at the very end - it causes sushi to crash with attempt to perform operations on NoneType.

This is a very hacky fix for this particular case

It would be better to fix handling of missing segments properly, but i do not understand sushi code to attempt that.