[Impeller] Fix SceneC crash for nodes with children (flutter#38346)

Author: bdero

impeller/fixtures/BUILD.gn

@@ -34,7 +34,10 @@ impeller_shaders("shader_fixtures") {
 }
 
 scenec("scene_fixtures") {
-  geometry = [ "flutter_logo.glb" ]
+  geometry = [
+    "flutter_logo.glb",
+    "two_triangles.glb",
+  ]
   type = "gltf"
 }
 
@@ -79,6 +82,7 @@ test_fixtures("file_fixtures") {
     "table_mountain_py.png",
     "table_mountain_pz.png",
     "test_texture.frag",
+    "two_triangles.glb",
     "types.h",
     "wtf.otf",
   ]

impeller/fixtures/two_triangles.glb

@@ -189,7 +189,7 @@ static void ProcessNode(const tinygltf::Model& gltf,
   /// Children.
   ///
 
-  for (size_t node_i = 0; node_i < out_node.children.size(); node_i++) {
+  for (size_t node_i = 0; node_i < in_node.children.size(); node_i++) {
     auto child = std::make_unique<fb::NodeT>();
     ProcessNode(gltf, gltf.nodes[in_node.children[node_i]], *child);
     out_node.children.push_back(std::move(child));