string.c: wchar succ

• string.c (enc_succ_char, enc_pred_char): consider wchar case.
  [ruby-core:56071] [Bug ]
• string.c (rb_str_succ): do not replace with invalid char.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42078 b2dd03c8-39d4-4d8f-98ff-823fe69b080e