path: avoid fruitless work when the target node is not reachable

path/a_star.go

@@ -33,7 +33,7 @@ type HeuristicCoster interface {
 // falling back to NullHeuristic otherwise. If the graph does not implement graph.Weighter,
 // graph.UniformCost is used. AStar will panic if g has an A*-reachable negative edge weight.
 func AStar(s, t graph.Node, g graph.Graph, h Heuristic) (path Shortest, expanded int) {
-	if !g.Has(s) {
+	if !g.Has(s) || !g.Has(t) {
 		return Shortest{from: s}, 0
 	}
 	var weight graph.WeightFunc