Only call close if the IO actually has a non-closed descriptor

* @descriptor is never null for us, either -1 (closed) or a valid fd.

Author: eregon

src/main/ruby/core/io.rb

@@ -1149,7 +1149,7 @@ def self.setup(io, fd, mode=nil, sync=false)
     end
 
     # Close old descriptor if there was already one associated
-    io.close if io.descriptor
+    io.close if io.descriptor != -1
 
     io.descriptor = fd
     io.mode       = mode || cur_mode