sdl_sound.cpp: Correct default device selection logic.

[vanfanel]

The logic on the SDL_OpenAudioDevice() call was inverted.
This:
SDL_OpenAudioDevice(dev.m_def ? nullptr : dev.m_name.c_str(), 0, &dspec, &ospec, 0);
...would mean that if dev.m_def has non-zero value, nullptr would be used instead of dev.m_name.c_str(), which makes no sense: we want to use dev.m_name.c_str() when dev.m_def is non-zero, and nullptr otherwise.

As a side-effect, this works around SDL2's ALSA backend inability to report a default audio device with SDL_GetDefaultAudioInfo() and thus is a workaround for issue #13910
That 's because, when no default device is reported by SDL_GetDefaultAudioInfo(), nullptr is properly passed to SDL_OpenAudioDevice() now and the default device is chosen by SDL2 as most SDL2 programs/games do.

[ajrhacker]

I strongly suspect this fix is technically incorrect and just masks the problem. It forces SDL to select the default device for any device not found through SDL_GetDefaultAudioInfo.