GHSA-qh5x-rfwf-rvfv: Hysteria vulnerable to server crash when max_datagram_frame_size very small

An authenticated client can crash the Hysteria server by advertising a very small QUIC max_datagram_frame_size and then triggering a UDP response from the server. When the server tries to send the UDP response back via QUIC DATAGRAM, quic-go returns DatagramTooLargeError. The server then attempts to fragment the Hysteria UDP message, but the fragmentation code does not handle the case where the UDP message header itself is larger than the maximum datagram payload size. This results in a slice bounds panic and terminates the server process.