fix: use db when it has been opened (#481)

src/server/pegasus_server_impl.cpp

@@ -414,7 +414,14 @@ void pegasus_server_impl::parse_checkpoints()
     }
 }
 
-pegasus_server_impl::~pegasus_server_impl() = default;
+pegasus_server_impl::~pegasus_server_impl()
+{
+    if (_is_open) {
+        dassert(_db != nullptr, "");
+        delete _db;
+        _db = nullptr;
+    }
+}
 
 void pegasus_server_impl::gc_checkpoints(bool force_reserve_one)
 {
@@ -1664,8 +1671,10 @@ void pegasus_server_impl::on_clear_scanner(const int64_t &args) { _context_cache
 
 void pegasus_server_impl::cancel_background_work(bool wait)
 {
-    dassert(_db != nullptr, "");
-    rocksdb::CancelAllBackgroundWork(_db, wait);
+    if (_is_open) {
+        dassert(_db != nullptr, "");
+        rocksdb::CancelAllBackgroundWork(_db, wait);
+    }
 }
 
 ::dsn::error_code pegasus_server_impl::stop(bool clear_state)