gh-131311: Fix additional memory leaks in ctypes (GH-131429)

* Visit keep in StructParam_traverse
* Decref swapped in PyCSimpleType_init
* Decref ob in make_funcptrtype_dict
* Check Pointer_item result while constructing result list in Pointer_subscript

* Fix align and size naming in PyCStructUnionType_update_stginfo
  - as discussed in previous PR