Initialize the return slot for small return values from callbacks.
Initialize the return slot for small return values from callbacks.

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