Fix incorrect D3D11 assert occasionally triggering in StagingTexture

The texture had to be a compressed cubemap or texture array to trigger,
and the conditions were order dependant (since "if( mWidth < blockWidth
|| mHeight < blockHeight )" conditional needs to be met)

RenderSystems/Direct3D11/src/OgreD3D11StagingTexture.cpp

@@ -243,7 +243,7 @@ namespace Ogre
             // There can and should only be one record.
             if( mWidth < blockWidth || mHeight < blockHeight )
             {
-                OGRE_ASSERT_LOW( mFreeBoxes.size() == 1u );
+                OGRE_ASSERT_LOW( mFreeBoxes[slice].size() == 1u );
                 canShrink = false;
             }
         }