Fix race in reload and gateway sublist check

Signed-off-by: Waldemar Quevedo <wally@nats.io>

server/gateway.go

@@ -2680,12 +2680,11 @@ func (s *Server) gatewayHandleSubjectNoInterest(c *client, acc *Account, accName
 	// If there is no subscription for this account, we would normally
 	// send an A-, however, if this account has the internal subscription
 	// for service reply, send a specific RS- for the subject instead.
-	hasSubs := acc.sl.Count() > 0
-	if !hasSubs {
-		acc.mu.RLock()
-		hasSubs = acc.siReply != nil
-		acc.mu.RUnlock()
-	}
+	// Need to grab the lock here since sublist can change during reload.
+	acc.mu.RLock()
+	hasSubs := acc.sl.Count() > 0 || acc.siReply != nil
+	acc.mu.RUnlock()
+
 	// If there is at least a subscription, possibly send RS-
 	if hasSubs {
 		sendProto := false