gracefully handle Initial+Handshake packets after dropping Initial epoch

When datagram with Initial+Handshake coalesced packets is received after
the Initial epoch has already been dropped, we would drop the entire
datagram, causing the peer to resend the Handshake packet.

Instead we should just skip over the Initial packet and continue
processing the datagram.

Closes #1834.

quiche/src/lib.rs

@@ -2654,6 +2654,17 @@ impl Connection {
                     return Ok(pkt_len);
                 }
 
+                // A datagram with Initial+Handshake coalesced packets might be
+                // received after the Initial epoch has already been dropped.
+                //
+                // To avoid rejecting the Handshake packet (due to the Initial
+                // being undecryptable), simply skip over the Initial packet and
+                // continue processing the rest of the datagram.
+                if hdr.ty == packet::Type::Initial {
+                    let pkt_len = b.off() + payload_len;
+                    return Ok(pkt_len);
+                }
+
                 let e = drop_pkt_on_err(
                     Error::CryptoFail,
                     self.recv_count,