fix index out of bounds

Author: lrvideckis

library/trees/uncommon/count_paths_per_length.hpp

@@ -9,7 +9,7 @@
 //! which are each O(n)
 vector<ll> count_paths_per_length(const vector<vi>& adj) {
   vector<ll> num_paths(sz(adj));
-  num_paths[1] = sz(adj) - 1;
+  if (sz(adj) >= 2) num_paths[1] = sz(adj) - 1;
   edge_cd(adj,
     [&](const vector<vi>& cd_adj, int cent, int split) {
       vector<vector<double>> cnt(2, vector<double>(1));