fix npe

Author: zstan

modules/core/src/main/java/org/apache/ignite/internal/processors/cache/distributed/dht/GridDhtTxFinishFuture.java

@@ -702,7 +702,7 @@ private void onNodeLeft(UUID nodeId) {
                         for (UUID backupId : backups) {
                             ClusterNode backup = cctx.discovery().node(backupId);
 
-                            if (!backup.isLocal()) {
+                            if (backup != null && !backup.isLocal()) {
                                 if (salvageReq == null)
                                     salvageReq = new GridDhtTxSalvageMessage(tx.nearXidVersion());