Fixes an error with the CoreAudio backend, which apparently doesn't
allow you to set the volume before starting the stream:
```
59:31:087 AudioCommon/CoreAudioSoundStream.cpp:97 E[Audio]: error setting volume
```
This shouldn't cause any problems with other backends, since the mixer
starts with silence anyways.
The NullAudio backend is guaranteed to be compiled in, so no reason
to check it.
In addition to that, if it wasn't valid, it wouldn't work as a fallback
in InitSoundStream as there are uses to g_sound_stream later.
If the selected audio backend fails to Start() (which could happen for
example if there is no audio device), we currently still use the backend
anyway. This can lead to crashes on some platforms (such as Windows) and
is outright wrong anyway.
This commit fallbacks to the Null audio backend if the selected backend
couldn't be started.
This fixes bug #6001
We had to lock audiocommon with the old asynchron HLE audio emulation,
now our Mixer is just a plain FIFO which may underrun.
Of course, this will stutter, but underruning the audio backend is likely worse.
