dmband: Store the iface instead of the COM object in _DMUS_PRIVATE_BAND.

dlls/dmband/bandtrack.c

@@ -317,7 +317,7 @@ static HRESULT IDirectMusicBandTrack_IPersistStream_LoadBand (LPPERSISTSTREAM if
       return  E_OUTOFMEMORY;
     }
     pNewBand->BandHeader = *pHeader;
-    pNewBand->pBand = (IDirectMusicBandImpl*)((char*)(*ppBand) - offsetof(IDirectMusicBandImpl,BandVtbl));
+    pNewBand->band = *ppBand;
     IDirectMusicBand_AddRef(*ppBand);
     list_add_tail (&This->Bands, &pNewBand->entry);
   }

dlls/dmband/dmband_private.h

@@ -78,7 +78,7 @@ typedef struct _DMUS_PRIVATE_INSTRUMENT {
 typedef struct _DMUS_PRIVATE_BAND {
 	struct list entry; /* for listing elements */
 	DMUS_PRIVATE_BAND_ITEM_HEADER BandHeader;
-	IDirectMusicBandImpl* pBand;
+	IDirectMusicBand *band;
 } DMUS_PRIVATE_BAND, *LPDMUS_PRIVATE_BAND;