Bug 1719389 - Null check correct DrawTarget. r=miko

Differential Revision: https://phabricator.services.mozilla.com/D119241

layout/painting/nsDisplayList.cpp

@@ -6222,7 +6222,7 @@ void nsDisplayBlendMode::Paint(nsDisplayListBuilder* aBuilder,
 
   RefPtr<DrawTarget> temp =
       dt->CreateClippedDrawTarget(deviceRect, SurfaceFormat::B8G8R8A8);
-  if (!dt) {
+  if (!temp) {
     return;
   }