Fix invalid memory handling in C++ OBJ MTL Importer.

An ID created with regualr ID management code should never ever be
directly freed directly.

For embedded nodetrees, there is a dedicated function.

Reviewed By: aras_p

Differential Revision: https://developer.blender.org/D15827

source/blender/io/wavefront_obj/importer/obj_import_mtl.cc

@@ -143,6 +143,11 @@ static Image *load_texture_image(Main *bmain, const tex_map_XX &tex_map, bool re
   return image;
 }
 
+void UniqueNodetreeDeleter::operator()(bNodeTree *node)
+{
+  ntreeFreeEmbeddedTree(node);
+}
+
 ShaderNodetreeWrap::ShaderNodetreeWrap(Main *bmain,
                                        const MTLMaterial &mtl_mat,
                                        Material *mat,

source/blender/io/wavefront_obj/importer/obj_import_mtl.hh

@@ -22,10 +22,7 @@
 namespace blender::io::obj {
 
 struct UniqueNodetreeDeleter {
-  void operator()(bNodeTree *node)
-  {
-    MEM_freeN(node);
-  }
+  void operator()(bNodeTree *node);
 };
 
 using unique_nodetree_ptr = std::unique_ptr<bNodeTree, UniqueNodetreeDeleter>;