Bug 1373229: P3. Ensure that the retrieval operation succeeded. r=bas

Retrieving ID3D10Multithread can fail (and will do so on Win7 try box). We must check for null before using the object.

MozReview-Commit-ID: LTeBgDwR2EF

--HG--
extra : rebase_source : d108452fd87fe0e231adee471347df52a67a6c92

gfx/thebes/DeviceManagerDx.cpp

@@ -598,9 +598,9 @@ DeviceManagerDx::CreateDecoderDevice()
 
   RefPtr<ID3D10Multithread> multi;
   device->QueryInterface(__uuidof(ID3D10Multithread), getter_AddRefs(multi));
-
-  multi->SetMultithreadProtected(TRUE);
-
+  if (multi) {
+    multi->SetMultithreadProtected(TRUE);
+  }
   if (reuseDevice) {
     mDecoderDevice = device;
   }