Fix point instancer crash

Apply a fix similar to other palces in the same file. Not 100% it is
entirely right, as my knowledge of the VP2 render delegate and instancer
is not high.

Author: pierrebai-adsk

lib/mayaUsd/render/vp2RenderDelegate/mayaPrimCommon.cpp

@@ -778,7 +778,15 @@ void MayaUsdRPrim::_SyncDisplayLayerModes(SdfPath const& id, bool instancedPrim)
     // Obtain scene prim path
     HdInstancerContext instancerContext;
     int                instanceIndex = instancedPrim ? 0 : UsdImagingDelegate::ALL_INSTANCES;
-    auto               usdPath = drawScene.GetScenePrimPath(id, instanceIndex, &instancerContext);
+
+    auto delegate = drawScene.GetUsdImagingDelegate();
+    auto instancerId = delegate->GetInstancerId(id);
+    bool instanced = !instancerId.IsEmpty();
+    // The additional condition below is to prevent a crash in USD function GetScenePrimPath
+    instanced = (instanced && !delegate->GetInstanceIndices(instancerId, id).empty());
+
+    auto usdPath
+        = instanced ? drawScene.GetScenePrimPath(id, instanceIndex, &instancerContext) : SdfPath();
 
     // Native instances use per-instance _displayLayerModes
     if (instancedPrim && instancerContext.empty()) {