thread_pthread.c: use get_stack

* thread_pthread.c (ruby_init_stack, ruby_stack_overflowed_p):
  place get_stack above others to get stack boundary information.
  [ruby-core:60113] [Bug #9454]

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44726 b2dd03c8-39d4-4d8f-98ff-823fe69b080e