audio: retain WASAPI clients only after initialization (#807)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #782 

## Description of change

This addresses an issue only seen on Linux + WINE.

Defer retaining WASAPI clients until `Initialize` succeeds.

Before this PR, temporary clients created during DirectShow device
capability probing were retained as active audio clients, causing
Wine/Linux crashes in DDR and popn.

Also covers IAudioClient3, dummy, ASIO, WaveOut, and null-device
initialization paths, for consistency.

## Testing
WIP

src/spice2x/hooks/audio/backends/mmdevice/null_device.cpp

@@ -153,19 +153,9 @@ HRESULT STDMETHODCALLTYPE NullMMDevice::Activate(
     log_info("audio::null", "NullMMDevice::Activate {}", guid2s(iid));
 
     if (iid == IID_IAudioClient) {
-
-        // release any previously persisted client
-        if (hooks::audio::CLIENT != nullptr) {
-            hooks::audio::CLIENT->Release();
-        }
-
         auto *client = static_cast<IAudioClient *>(new DummyIAudioClient(new NullDiscardBackend()));
         *ppInterface = client;
 
-        // persist the audio client
-        hooks::audio::CLIENT = client;
-        hooks::audio::CLIENT->AddRef();
-
         return S_OK;
     }