Fix potential memory overwrite in sexp_bignum_fxmul. Patch from Lorenzo.
Fix potential memory overwrite in sexp_bignum_fxmul.
Patch from Lorenzo.

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