message: Check iterator is valid in the shortest path search
  Test Plan: all tests pass
  Reviewers: davidn, jakeh
diff: @@ -162,6 +162,7 @@ class MetaGraph {
  visited.insert(top);
  const auto itr = graph_.find(top);
  + if (itr != graph_.end()) {
  std::for_each(itr->second.begin(), itr->second.end(), [&](T v) {
  if (!visited.contains(v)) {
  dist[v] = dist[top] + 1;
  @@ -170,6 +171,7 @@ class MetaGraph {
  }
  });
  }
  + }
  std::vector<T> path = {dst};
  while (path.back() != src) {
  