Make graph.Delta zero state invalid for use

Previously, an incorrectly initialised Delta (omission of Action) would
result in an Add operation. Make that detectable and return an error.

graph/leveldb/quadstore.go

@@ -185,6 +185,9 @@ func (qs *QuadStore) ApplyDeltas(deltas []graph.Delta) error {
 	resizeMap := make(map[string]int64)
 	sizeChange := int64(0)
 	for _, d := range deltas {
+		if d.Action != graph.Add && d.Action != graph.Delete {
+			return errors.New("leveldb: invalid action")
+		}
 		bytes, err := json.Marshal(d)
 		if err != nil {
 			return err