Async Reader: Initialize a member variable in the constructor.

Fixes CID 966379.
CBaseDispatch: Remove duplicated sanity check.

Fixes CID 1224558.
BaseClasses/wxutil.cpp: Ensure freed memory can't be reused by mistake.

Fixes CID 1224550.
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...