[release/1.3.4] eth/downloader: fix header download limiting

Fixes ethereum#2201
(cherry picked from commit 26e72b2)

Author: fjl

eth/downloader/downloader.go

@@ -1216,7 +1216,7 @@ func (d *Downloader) fetchHeaders(p *peer, td *big.Int, from uint64) error {
 				}
 			}
 			// Notify the content fetchers of new headers, but stop if queue is full
-			cont := d.queue.PendingBlocks() < maxQueuedHeaders || d.queue.PendingReceipts() < maxQueuedHeaders
+			cont := d.queue.PendingBlocks() < maxQueuedHeaders && d.queue.PendingReceipts() < maxQueuedHeaders
 			for _, ch := range []chan bool{d.bodyWakeCh, d.receiptWakeCh, d.stateWakeCh} {
 				if cont {
 					// We still have headers to fetch, send continuation wake signal (potential)