Improve comment and assertion

Author: David Roberts

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/dataframe/transforms/DataFrameTransformStats.java

@@ -258,7 +258,8 @@ public static State fromComponents(DataFrameTransformTaskState taskState, Indexe
                 return FAILED;
             } else {
 
-                // Note: indexer state CAN be null if the task state is stopped or failed
+                // If we get here then the task state must be started, and that means we should have an indexer state
+                assert(taskState == DataFrameTransformTaskState.STARTED);
                 assert(indexerState != null);
 
                 switch (indexerState) {