vm.c: initialize root_svar

* vm.c (th_init, ruby_thread_init): initialize root_svar with Qnil,
  since lep_svar_place() expects uninitialized svar to be nil, not 0.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38748 b2dd03c8-39d4-4d8f-98ff-823fe69b080e