parse.y: junk sigil only names

* parse.y (intern_str): sigil only names are junk, at least one
  identifier character is needed.  [ruby-dev:47723] [Bug #8928]
* parse.y (rb_enc_symname_type): fix out of bound access.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42988 b2dd03c8-39d4-4d8f-98ff-823fe69b080e