Guard against bad edges being added to topology

birdvisu/topo_v3.py

@@ -78,6 +78,7 @@ class TopologyV3:
 
 	def add_edge(self, edge):
 		if self.frozen: raise ValueError('Cannot add edge to frozen topology.')
+		if edge.count <= 0: raise ValueError('Must not add non-existent edges')
 		# We need to merge relevant edges. Thus, this is more like adding a
 		# multi-edge, which falls to no-op when the specific edge is already
 		# present. In other words, this means that we _may_ have multiple Edge