optimize the judge of invalide notification.number (ethereum#22658)

Don't bother fetching genesis

Co-authored-by: wuff1996 <33193253+wuff1996@users.noreply.github.com>

eth/fetcher/block_fetcher.go

@@ -391,13 +391,14 @@ func (f *BlockFetcher) loop() {
 				blockAnnounceDOSMeter.Mark(1)
 				break
 			}
+			if notification.number == 0 {
+				break
+			}
 			// If we have a valid block number, check that it's potentially useful
-			if notification.number > 0 {
-				if dist := int64(notification.number) - int64(f.chainHeight()); dist < -maxUncleDist || dist > maxQueueDist {
-					log.Debug("Peer discarded announcement", "peer", notification.origin, "number", notification.number, "hash", notification.hash, "distance", dist)
-					blockAnnounceDropMeter.Mark(1)
-					break
-				}
+			if dist := int64(notification.number) - int64(f.chainHeight()); dist < -maxUncleDist || dist > maxQueueDist {
+				log.Debug("Peer discarded announcement", "peer", notification.origin, "number", notification.number, "hash", notification.hash, "distance", dist)
+				blockAnnounceDropMeter.Mark(1)
+				break
 			}
 			// All is well, schedule the announce if block's not yet downloading
 			if _, ok := f.fetching[notification.hash]; ok {