thread.c: Mutex#synchronize no block params

* thread.c (rb_mutex_synchronize_m): yield no block params.  patch by
  splattael (Peter Suschlik) in [ruby-core:53773] [Bug #8097].
  fix GH-266.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40000 b2dd03c8-39d4-4d8f-98ff-823fe69b080e