release VM stack properly.

	* cont.c: r55766 change the handling method of Fiber's VM stack.
	  Resumed Fiber points NULL as VM stack and running Thread has
	  responsibility to manage it (marking and releasing).

	  However, thread_start_func_2()@thread.c and thread_free()@vm.c
	  doesn't free the VM stack if corresponding root Fiber is exist.
	  This causes memory leak. [Bug #13772]

	* cont.c (root_fiber_alloc): fib->cont.saved_thread.ec.stack should be NULL
	  because running thread has responsibility to manage this stack.

	* vm.c (rb_thread_recycle_stack_release): assert given stack is not NULL
	  (callers should care it).

	fix stack storing for root fibers.

	* cont.c (root_fiber_alloc): this function is called by fiber_current()
	  and fiber_store(). fiber_current() should clear VM stack information
	  in a fiber data because runnning thread knows stack information and has
	  responsibility to manage it. However fiber_store() requires to remain
	  VM stack information in a fiber data because the responsibility to manage
	  VM stack is moved to the Fiber from the Thread (and switch to another
	  fiber).

	* cont.c (root_fiber_alloc): save thread's fiber and root_fiber information.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59516 b2dd03c8-39d4-4d8f-98ff-823fe69b080e