miner: exit loop when downloader Done or Failed

Following the logic of the comment at the method,
this fixes a regression introduced at 7cf56d6f06
, which would allow external parties to DoS with
blocks, preventing mining progress.

Signed-off-by: meows <b5c6@protonmail.com>

miner/miner.go

@@ -111,6 +111,8 @@ func (miner *Miner) update() {
 					miner.SetEtherbase(miner.coinbase)
 					miner.worker.start()
 				}
+				// stop immediately and ignore all further pending events
+				return
 			}
 		case addr := <-miner.startCh:
 			if canStart {