2 corner case tests fail with ZSH

[smondet]

(only with zsh)

    exits 27 Construct.(
        if_then_else (
          (getenv (string "HOME\000ME")) =$= string (Sys.getenv "HOME")
        )
          (return 12) (return 27)
      );

Fails with 12.

    exits 0 Construct.(
        let var = string "VVVVVVV" in
        let assert_or_return ret cond =
          if_then_else cond nop (seq [printf "Fail: %d" ret; fail]) in
        seq [
(* ... *)
          setenv ~var (string "Bouhoo\000bah\n");
          assert_or_return 12 (getenv var =$= string "Bouhoobah");
(* ... *)
      );

fails (this is undefined behavior anyway).

[smondet]

The Schism of The String Types fixed this a while ago.