[release-v2.0] p2p: Disconnect from peers that sent too few headers

The existing full node software (dcrd) always sends batches of 2000
headers when requesting headers during Initial Block Download.

The check added in this commit ensures the wallet disconnects from any
peers that deviate from this behavior. While receiving less than 2000
headers could not cause any consistency or safety issues, it could slow
down sync time.

Backport of 34395a5.

p2p/peering.go

@@ -1121,6 +1121,22 @@ func (rp *RemotePeer) deleteRequestedHeaders() {
 func (rp *RemotePeer) receivedHeaders(ctx context.Context, msg *wire.MsgHeaders) {
 	const opf = "remotepeer(%v).receivedHeaders"
 	rp.requestedHeadersMu.Lock()
+
+	// Ensure the remote peer sent as many headers as it could. It can only
+	// send fewer than 2k headers when the last one is >= their advertised
+	// height (their tip height). This handles cases where a peer might
+	// drip headers instead of sending a full batch.
+	tooFewHeaders := len(msg.Headers) > 0 &&
+		len(msg.Headers) < wire.MaxBlockHeadersPerMsg &&
+		msg.Headers[len(msg.Headers)-1].Height < uint32(rp.initHeight)
+	if tooFewHeaders {
+		op := errors.Opf(opf, rp.raddr)
+		err := errors.E(op, errors.Protocol, "peer sent too few headers")
+		rp.Disconnect(err)
+		rp.requestedHeadersMu.Unlock()
+		return
+	}
+
 	var prevHash chainhash.Hash
 	var prevHeight uint32
 	for i, h := range msg.Headers {