* thread.c (Init_Thread): ditto.
* thread.c (rb_vm_gvl_destroy): ditto.

* thread.c (thread_start_func_2): make sure vm->running_thread
  don't point to dead thread.
* thread.c (timer_thread_function): close a race against thead
  destruction. [Bug #4911][ruby-dev:43859]

* vm_core.h (rb_thread_set_current): reset running time of
  current thread instead of previous thread. We no longer
  assume previous running thread still live.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38063 b2dd03c8-39d4-4d8f-98ff-823fe69b080e