* string.c (rb_str_rpartition): revert r39903, and convert byte offset
  to char offset; the return value of rb_reg_search is byte offset,
  but other than it of rb_str_rpartition expects char offset.
  [Bug #8138] [ruby-dev:47183]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39906 b2dd03c8-39d4-4d8f-98ff-823fe69b080e