hash.c: use rb_check_arity

* hash.c (rb_hash_initialize, rb_hash_fetch_m, rb_hash_default):
  use rb_check_arity over rb_scan_args.
  (env_fetch): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46660 b2dd03c8-39d4-4d8f-98ff-823fe69b080e