DVB: Add a special case for SetChannel at first start or after clearing all channels.

We don't want to assert in this case since it's expected that we have no channel to set.

Also make nDVBLastChannel a signed integer for consistency with the rest of our API.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
WebUI: Inform the web interface that no audio stream is available.