VobSub: Correctly check for allocation errors.
VobSub: Use size_t when it makes more sense than using int.
Comment out some unused variable assignments.

Those can be useful if the functions are extended one day.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
Why "C:\tmp.srt"?
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...