making sure time is not negative

Author: masseyke

x-pack/plugin/migrate/src/main/java/org/elasticsearch/xpack/migrate/task/ReindexDataStreamPersistentTaskExecutor.java

@@ -366,6 +366,11 @@ private TimeValue updateCompletionTimeAndGetTimeToLive(
     }
 
     private TimeValue getTimeToLive(long completionTimeInMillis) {
-        return TimeValue.timeValueMillis(TASK_KEEP_ALIVE_TIME.millis() - (threadPool.absoluteTimeInMillis() - completionTimeInMillis));
+        return TimeValue.timeValueMillis(
+            TASK_KEEP_ALIVE_TIME.millis() - Math.max(
+                TASK_KEEP_ALIVE_TIME.millis(),
+                threadPool.absoluteTimeInMillis() - completionTimeInMillis
+            )
+        );
     }
 }