GC-preserving intermediate sexp_fixnum_to_bignum result in sexp_expt. Patch from Lorenzo.
GC-preserving intermediate sexp_fixnum_to_bignum result in sexp_expt.
Patch from Lorenzo.

norton/chibi-scheme,norton/chibi-scheme,holmescn/chibi-scheme,norton/chibi-scheme,norton/chibi-scheme,holmescn/chibi-scheme,holmescn/chibi-scheme,holmescn/chibi-scheme,norton/chibi-scheme,holmescn/chibi-scheme,norton/chibi-scheme