[Serialization] Better error message when idx_list out of bound (#2848)

Co-authored-by: Minjie Wang <wmjlyjemaine@gmail.com>

src/graph/serialize/dglgraph_serialize.cc

@@ -163,7 +163,11 @@ StorageMetaData LoadDGLGraphs(const std::string &filename,
     // Would be better if idx_list is sorted. However the returned the graphs
     // should be the same order as the idx_list
     for (uint64_t i = 0; i < idx_list.size(); ++i) {
-      fs->Seek(graph_indices[idx_list[i]]);
+      auto gid = idx_list[i];
+      CHECK((gid < graph_indices.size()) && (gid >= 0))
+        << "ID " << gid
+        << " in idx_list is out of bound. Please check your idx_list.";
+      fs->Seek(graph_indices[gid]);
       GraphData gdata = GraphData::Create();
       GraphDataObject *gdata_ptr =
         const_cast<GraphDataObject *>(gdata.as<GraphDataObject>());

src/graph/serialize/heterograph_serialize.cc

@@ -169,7 +169,11 @@ std::vector<HeteroGraphData> LoadHeteroGraphs(const std::string &filename,
     // Would be better if idx_list is sorted. However the returned the graphs
     // should be the same order as the idx_list
     for (uint64_t i = 0; i < idx_list.size(); ++i) {
-      fs->Seek(graph_indices[idx_list[i]]);
+      auto gid = idx_list[i];
+      CHECK((gid < graph_indices.size()) && (gid >= 0))
+        << "ID " << gid
+        << " in idx_list is out of bound. Please check your idx_list.";
+      fs->Seek(graph_indices[gid]);
       HeteroGraphData gdata = HeteroGraphData::Create();
       auto hetero_data = gdata.sptr();
       fs->Read(&hetero_data);