QUIC: fixed padding of initial packets in case of limited path. Previously, non-padded initial packet could be sent as a result of the following situation: - initial queue is not empty (so padding to 1200 is required) - handshake queue is not empty (so padding is to be added after h/s packet) - path is limited If serializing handshake packet would violate path limit, such packet was omitted, and the non-padded initial packet was sent. The fix is to avoid sending the packet at all in such case. This follows the original intention introduced in c5155a0cb12f.