Bug 1126918 - NewShSurfaceHandle can return null. Guard against it. r=jgilbert

gfx/layers/client/CanvasClient.cpp

@@ -333,6 +333,9 @@ static TemporaryRef<gl::ShSurfHandle>
 CloneSurface(gl::SharedSurface* src, gl::SurfaceFactory* factory)
 {
     RefPtr<gl::ShSurfHandle> dest = factory->NewShSurfHandle(src->mSize);
+    if (!dest) {
+        return nullptr;
+    }
     SharedSurface::ProdCopy(src, dest->Surf(), factory);
     return dest.forget();
 }