encoding.c: revert r41964

* encoding.c (enc_set_index): since r41967, old terminator is dealt
  with in str_fill_term().  should not consider it here because this
  function is called before any encoding is set.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41994 b2dd03c8-39d4-4d8f-98ff-823fe69b080e