class.c: allow nil keyword_hash

* class.c (rb_extract_keywords): treat nil keyword_hash same as 0,
  for the case rb_scan_args returns nil if no keyword hash.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44702 b2dd03c8-39d4-4d8f-98ff-823fe69b080e