Decrease the default level of try_fdopen.

Because it cause Bus Error:
http://u32.rubyci.org/~chkbuild/ruby-trunk/log/20120407T230202Z.log.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@35260 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_io.rb

@@ -1455,7 +1455,7 @@ class TestIO < Test::Unit::TestCase
     f.close
   end
 
-  def try_fdopen(fd, autoclose = true, level = 100)
+  def try_fdopen(fd, autoclose = true, level = 50)
     if level > 0
       begin
         1.times {return try_fdopen(fd, autoclose, level - 1)}