vm.c: initialize defined_module_hash early

* vm.c (Init_BareVM): initialize defined_module_hash here,
  Init_top_self() is too late to register core classes/modules.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43273 b2dd03c8-39d4-4d8f-98ff-823fe69b080e