UsdUfe: Verify layers to prevent potential dereferencing of null hand…

…le during save.

While UsdStage::GetUsedLayers() is not expected to return null layers,
an edge case was observed in production scenes using valueClips with
auto-generated manifest layers.

lib/usdUfe/utils/layers.cpp

@@ -95,7 +95,7 @@ StageDirtyState isStageDirty(const PXR_NS::UsdStage& stage)
 
     SdfLayerHandleVector allLayers = stage.GetUsedLayers(true);
     for (auto layer : allLayers) {
-        if (!layer->IsDirty())
+        if (!TF_VERIFY(layer) || !layer->IsDirty())
             continue;
 
         if (rootLayers.count(layer))