conn: clear system handlers on reconnect

xmpp_conn_t object can be reused with saving user's handlers.
However, saving system handlers can lead to a fail during connection
process. This is because old object may contain timed handlers for
missed features or other handlers that would handle incoming stanzas
incorrectly.

src/conn.c

@@ -1215,6 +1215,8 @@ static void _conn_reset(xmpp_conn_t * const conn)
     conn->secured = 0;
     conn->tls_failed = 0;
     conn->error = 0;
+
+    handler_system_delete_all(conn);
 }
 
 static int _conn_connect(xmpp_conn_t * const conn,