Don't call node.addIntf() if node is None

fixes #716

mininet/link.py

@@ -50,6 +50,7 @@ class Intf( object ):
             self.ip = '127.0.0.1'
             self.prefixLen = 8
         # Add to node (and move ourselves if necessary )
+        if node:
             moveIntfFn = params.pop( 'moveIntfFn', None )
             if moveIntfFn:
                 node.addIntf( self, port=port, moveIntfFn=moveIntfFn )