Prevent number of remote write shards from going negative.

This can happen in the situation where the system scales up the number of shards massively (to deal with some backlog), then scales it down again as the number of samples sent during the time period is less than the number received.

storage/remote/queue_manager.go

@@ -339,6 +339,8 @@ func (t *QueueManager) calculateDesiredShards() {
 	numShards := int(math.Ceil(desiredShards))
 	if numShards > t.cfg.MaxShards {
 		numShards = t.cfg.MaxShards
+	} else if numShards < 1 {
+		numShards = 1
 	}
 	if numShards == t.numShards {
 		return