vm_insnhelper.c: non-symbol key

* vm_insnhelper.c (vm_callee_setup_keyword_arg): non-symbol key is not
  a keyword argument, keep it as an positional argument.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40260 b2dd03c8-39d4-4d8f-98ff-823fe69b080e