Merge pull request #2973 from tomwilkie/2969-negative-shards

Prevent number of remote write shards from going negative.

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