rhoruby: check len pointer instead of val in _to_str_ex
rhoruby: check len pointer instead of val in _to_str_ex

UIKit0/rhodes,UIKit0/rhodes,UIKit0/rhodes,louisatome/rhodes,UIKit0/rhodes,louisatome/rhodes,rhosilver/rhodes-1,louisatome/rhodes,louisatome/rhodes,UIKit0/rhodes,UIKit0/rhodes,rhosilver/rhodes-1,rhosilver/rhodes-1,rhosilver/rhodes-1,rhosilver/rhodes-1,rhosilver/rhodes-1,louisatome/rhodes,UIKit0/rhodes,louisatome/rhodes,rhosilver/rhodes-1,rhosilver/rhodes-1,rhosilver/rhodes-1,louisatome/rhodes,louisatome/rhodes,UIKit0/rhodes,rhosilver/rhodes-1,UIKit0/rhodes,UIKit0/rhodes,louisatome/rhodes