This adds several fixes to make the event and menu music more robust:
1. The game will now run with zero soundtracks. All accesses into a potentially empty vector should now have checks.
2. Soundtracks and menu music are now checked for validity before being played (not just when importing a FSM mission).
3. The parsing validity checks and the in-game event music now check for skippable patterns using the same function.
4. Zero measures and zero samples-per-measure now cause the game to skip a pattern. Negative values now trigger a warning.
5. Omitting some or all of the `$Name:` lines for a soundtrack will not cause an assert.
Fixes#5576. Follow-up to #5572.
Goober5000
6a550a6701