object.c (rb_class_real): do not dereference 0 VALUE

* test/ruby/test_module.rb (test_inspect_segfault):
  Test case and bug report by Thomas Stratmann.
  [ruby-core:65214] [Bug #10282]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47715 b2dd03c8-39d4-4d8f-98ff-823fe69b080e