thread_pthread.c: cast to size_t

* thread_pthread.c (ruby_stack_overflowed_p): range of rlim_cur
  may be larger than size_t.  [ruby-core:60113] [Bug #9454]

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44715 b2dd03c8-39d4-4d8f-98ff-823fe69b080e