RUBY-433 prevent additonal possible IOError

Author: TylerBrock

lib/mongo/connection.rb

@@ -629,7 +629,9 @@ def check_is_master(node)
       rescue OperationFailure, SocketError, SystemCallError, IOError
         close
       ensure
-        socket.close if socket
+        if socket
+          socket.close unless socket.closed?
+        end
       end
 
       config