[release/1.3.4] eth/downloader: fix premature exit before notifying all part fetchers

(cherry picked from commit 64ee576)

Author: karalabe

eth/downloader/downloader.go

@@ -1230,9 +1230,11 @@ func (d *Downloader) fetchHeaders(p *peer, td *big.Int, from uint64) error {
 					case ch <- false:
 					case <-d.cancelCh:
 					}
-					return nil
 				}
 			}
+			if !cont {
+				return nil
+			}
 			// Queue not yet full, fetch the next batch
 			from += uint64(len(headers))
 			getHeaders(from)