dmusic: Improve IDirectMusicPortImpl_DownloadInstrument stub.

dlls/dmusic/port.c

@@ -88,8 +88,13 @@ static HRESULT WINAPI IDirectMusicPortImpl_Read (LPDIRECTMUSICPORT iface, LPDIRE
 
 static HRESULT WINAPI IDirectMusicPortImpl_DownloadInstrument (LPDIRECTMUSICPORT iface, IDirectMusicInstrument* pInstrument, IDirectMusicDownloadedInstrument** ppDownloadedInstrument, DMUS_NOTERANGE* pNoteRanges, DWORD dwNumNoteRanges) {
 	IDirectMusicPortImpl *This = (IDirectMusicPortImpl *)iface;
+
 	FIXME("(%p, %p, %p, %p, %d): stub\n", This, pInstrument, ppDownloadedInstrument, pNoteRanges, dwNumNoteRanges);
-	return S_OK;
+
+	if (!pInstrument || !ppDownloadedInstrument || (dwNumNoteRanges && !pNoteRanges))
+		return E_POINTER;
+
+	return DMUSIC_CreateDirectMusicDownloadedInstrumentImpl(&IID_IDirectMusicDownloadedInstrument, (LPVOID*)ppDownloadedInstrument, NULL);
 }
 
 static HRESULT WINAPI IDirectMusicPortImpl_UnloadInstrument (LPDIRECTMUSICPORT iface, IDirectMusicDownloadedInstrument *pDownloadedInstrument) {