class.c: always clear tables first

* class.c (rb_mod_init_copy): always clear instance variable,
  constant and method tables first, regardless the source tables.
  [ruby-dev:48182] [Bug #9813]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45874 b2dd03c8-39d4-4d8f-98ff-823fe69b080e