Zero-length DATAGRAM frames bypass the receive buffer limit in quinn-proto

Summary

quinn-proto bounds its queue of received datagrams by charging each queued frame's payload length against `TransportConfig::datagram_receive_buffer_size`, evicting the oldest datagrams when the limit is exceeded. A DATAGRAM frame with an empty payload was charged zero bytes, so it never triggered eviction and was appended to the queue, which has no other bound.

DATAGRAM frames are not subject to flow control. A peer that has completed the handshake can therefore send a stream of empty DATAGRAM frames and grow the receiver's heap without limit until the process is killed. An application that reads incoming datagrams but does so more slowly than the peer sends them is still affected. Receiving datagrams is enabled by default, so all endpoints that do not explicitly set `datagram_receive_buffer_size(None)` are affected.

The send path had the same accounting flaw: an application could enqueue an unbounded number of empty outgoing datagrams without ever receiving `SendDatagramError::Blocked`.

The flaw was corrected in 0.11.17 by accounting for the fixed per-datagram overhead in both directions, so that every queued datagram consumes buffer space.

Thanks to [@alexpyattaev](https://github.com/alexpyattaev) for reporting this issue.