parse.y: check for symbol

* parse.y (rb_sym2id, rb_sym2id_without_pindown): return 0 for
  non-symbol values, for the time being.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45700 b2dd03c8-39d4-4d8f-98ff-823fe69b080e