Bug 1864988 - Don't call CreateDataSourceSurfaceByCloning on null. r=gfx-reviewers,aosmond

Differential Revision: https://phabricator.services.mozilla.com/D194936

dom/canvas/CanvasRenderingContext2D.cpp

@@ -6003,7 +6003,9 @@ nsresult CanvasRenderingContext2D::GetImageDataArray(
   if (!usePlaceholder &&
       ShouldResistFingerprinting(RFPTarget::CanvasRandomization)) {
     needRandomizePixels = true;
-    readback = CreateDataSourceSurfaceByCloning(readback);
+    if (readback) {
+      readback = CreateDataSourceSurfaceByCloning(readback);
+    }
   }
 
   DataSourceSurface::MappedSurface rawData;