Fix handling of union tail padding.
Fix handling of union tail padding.

fdopen/ocaml-ctypes,yallop/ocaml-ctypes,dsheets/ocaml-ctypes,fdopen/ocaml-ctypes,ocamllabs/ocaml-ctypes,sjfloat/ocaml-ctypes,sjfloat/ocaml-ctypes,sjfloat/ocaml-ctypes,ocamllabs/ocaml-ctypes,whitequark/ocaml-ctypes,yallop/ocaml-ctypes,dsheets/ocaml-ctypes,dsheets/ocaml-ctypes,whitequark/ocaml-ctypes,whitequark/ocaml-ctypes,fdopen/ocaml-ctypes