Bug 846236 - Verify the buffer descriptor is a shared texture. r=ncameron, a=tef+

gfx/layers/opengl/CanvasLayerOGL.cpp

@@ -478,7 +478,7 @@ ShadowCanvasLayerOGL::GetLayer()
 LayerRenderState
 ShadowCanvasLayerOGL::GetRenderState()
 {
-  if (mDestroyed) {
+  if (mDestroyed || !IsValidSharedTexDescriptor(mFrontBufferDescriptor)) {
     return LayerRenderState();
   }
   return LayerRenderState(&mFrontBufferDescriptor,