Bug 614762: Null autoptr instead of release. r=jrmuizel a=blocking-beta8

gfx/layers/d3d9/DeviceManagerD3D9.cpp

@@ -522,7 +522,7 @@ DeviceManagerD3D9::VerifyReadyForRendering()
     mSwapChains[i]->Reset();
   }
 
-  mVB->Release();
+  mVB = nsnull;
   
   D3DPRESENT_PARAMETERS pp;
   memset(&pp, 0, sizeof(D3DPRESENT_PARAMETERS));