Stop sticky piece hasher from hashing new pieces after torrent close

Check t.closed in the pieceHasher loop before picking up new pieces.
Without this, the hasher goroutine could start hashing pieces against
already-closed storage after Drop(), causing "file already closed"
warnings visible in BenchmarkAddLargeTorrent.

Author: anacrolix

torrent.go

@@ -2784,6 +2784,9 @@ func (t *Torrent) startSinglePieceHasher() bool {
 func (t *Torrent) pieceHasher(initial pieceIndex) {
 	t.finishHash(initial)
 	for {
+		if t.closed.IsSet() {
+			break
+		}
 		piOpt := t.getPieceToHash()
 		if !piOpt.Ok {
 			break