Game exits with error when no audio hardware available

[DanRStevens]

This was noticed when investigating an issue for NAS2D:

• Re-introduced audio bug lairworks/nas2d-core#1103

When running OPHD in a VM without audio hardware available, the game exits on startup with an error message:

Music::load() error: Unrecognized audio format

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The later portion of the error comes from calling the sdl2-mixer function Mix_LoadMUS_RW, called from Music::Music:

Unrecognized audio format

This is despite the game having a fallback to MixerNull when it's unable to initialize MixerSDL.

Note that the Music class is one of the resource classes, and is unrelated to MixerSDL, even though it relies on sdl2-mixer.