objspace.c: newly added nodes

* ext/objspace/objspace.c (count_nodes): count also newly added nodes,
  and fix key for unknown node.  patch by tmm1 (Aman Gupta) in
  [ruby-core:53130] [Bug #8014]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39595 b2dd03c8-39d4-4d8f-98ff-823fe69b080e