lib: fix connection restore

PR-URL: #289
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Mykola Bilochub <nbelochub@gmail.com>

lib/session.js

@@ -83,7 +83,9 @@ class Session extends Map {
   }
 
   _restore(newConnection, receivedCount) {
-    this.connection.close();
+    if (this.connection) {
+      this.connection.close();
+    }
     this.connection = newConnection;
     for (let i = this.guaranteedDeliveredCount + 1; i <= receivedCount; i++) {
       this.buffer.delete(i);