Fix memory leak (#497)

Author: Sanhaoji2

include/abstract_data_store.h

@@ -18,7 +18,7 @@ template <typename data_t> class AbstractDataStore
   public:
     AbstractDataStore(const location_t capacity, const size_t dim);
 
-    // virtual ~AbstractDataStore() = default;
+    virtual ~AbstractDataStore() = default;
 
     // Return number of points returned
     virtual location_t load(const std::string &filename) = 0;

include/abstract_graph_store.h

@@ -18,6 +18,8 @@ class AbstractGraphStore
     {
     }
 
+    virtual ~AbstractGraphStore() = default;
+
     virtual int load(const std::string &index_path_prefix) = 0;
     virtual int store(const std::string &index_path_prefix) = 0;