storage: Fixed null pointer (#4130)

Fixes #4090

When trying to migrate a VM across 2 clusters, if a snapshot has been deleted and garbage collection has run to update the removed field, it is not possible to migrate the instance due to a null pointer.

Author: Spaceman1984

engine/storage/snapshot/src/main/java/org/apache/cloudstack/storage/snapshot/SnapshotDataFactoryImpl.java

@@ -73,6 +73,9 @@ public List<SnapshotInfo> getSnapshots(long volumeId, DataStoreRole role) {
         for (SnapshotDataStoreVO snapshotDataStoreVO : allSnapshotsFromVolumeAndDataStore) {
             DataStore store = storeMgr.getDataStore(snapshotDataStoreVO.getDataStoreId(), role);
             SnapshotVO snapshot = snapshotDao.findById(snapshotDataStoreVO.getSnapshotId());
+            if (snapshot == null){ //snapshot may have been removed;
+                continue;
+            }
             SnapshotObject info = SnapshotObject.getSnapshotObject(snapshot, store);
 
             infos.add(info);