Don't access sgSFX if sound is not initialized

[StephenCWills]

Fixes a crash in timedemo when running the test on an MSVC Debug build. Now that the sgSFX collection gets populated by a TSV file, it is not guaranteed to be populated and will in fact remain empty when running timedemo headless. I added some guards to prevent OOB access in two of the affected functions, but I find it's unclear what to do about GetSFXLength(). It makes me wonder if we should go ahead and load the TSV so we can have the option of testing towner dialogs with timedemo recordings.

[AJenbo]

I think we just return a hard coded time in that case or was that changed?

[StephenCWills]

We use a fixed value for FPS, but it's still multiplied by the length of the audio so different speeches would have different lengths. What has changed is we can no longer determine the length of the audio, but only in headless mode.