Fix Racket buffer loading code
Fix Racket buffer loading code

Should return #f on failure, not an empty buffer, for consistency with
Chez

mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm