transcode.c: fix segv in String.encode!

* transcode.c (str_transcode0): fix segv in String.encode!.  now
  rb_str_scrub() can return nil.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43504 b2dd03c8-39d4-4d8f-98ff-823fe69b080e