PackedScene: Prevent crash when root node has `parent` attribute

The crash happens further down when setting an invalid owner in
`Node::_set_owner_nocheck` but I couldn't figure out how to fix it.

But here the proper fix is to catch the invalid scene file early on
and fail loading it.

Part of #17372.

1 files changed, 3 insertions, 0 deletions

diff --git a/scene/resources/packed_scene.cpp b/scene/resources/packed_scene.cpp
index 058e89cf2e..cb201bc539 100644
--- a/scene/resources/packed_scene.cpp
+++ b/scene/resources/packed_scene.cpp
@@ -98,6 +98,9 @@ Node *SceneState::instance(GenEditState p_edit_state) const {
 			}
 #endif
 			parent = nparent;
+		} else {
+			// i == 0 is root node. Confirm that it doesn't have a parent defined.
+			ERR_FAIL_COND_V_MSG(n.parent != -1, nullptr, vformat("Invalid scene: root node %s cannot specify a parent node.", snames[n.name]));
 		}
 
 		Node *node = nullptr;