-1 crash, +1 improvement.

- Got rid of a crash in ScreenSelectMaster caused by GetTweenTimeLeft
when choosing an option that forces both players to choose it, i.e. in a
difficulty selection screen like in DDR Extreme, choosing Nonstop or
Oni.
Big props to Mad Matt for finding the error and indicating the fix.

- Changed the hardcoded PlayStartSound command when pressing Start a
second time to enter ScreenPlayerOptions after choosing a song. Now, a
separate sound can be used (conveniently named "ScreenSelectMusic enter
options").
If the file doesn't exist, no sound will play.