Add testcase for #14472 (#14921)

tests/arc/t14472.nim

@@ -0,0 +1,23 @@
+discard """
+  valgrind: true
+  cmd: "nim c --gc:arc -d:useMalloc $file"
+"""
+
+type
+  ImportMaterial* = object
+    # Adding a field here makes the problem go away.
+
+  Mesh* = object
+    vertices: seq[float32]
+    material: ImportMaterial
+
+  ImportedScene* = object
+    meshes*: seq[Mesh]
+
+proc bork() : ImportedScene =
+  var mats: seq[ImportMaterial]
+
+  setLen(mats, 1)
+  add(result.meshes, Mesh(material: mats[0]))
+
+var s = bork()