add exceptin in registerMerge && fixed test

lucene/core/src/java/org/apache/lucene/index/IndexWriter.java

@@ -4761,7 +4761,11 @@ public class IndexWriter
                   + segString(info)
                   + " does not exist in live infos");
         }
-        return false;
+        throw new MergePolicy.MergeException(
+            "MergePolicy selected a segment ("
+                + info.info.name
+                + ") that is not in the current index "
+                + segString());
       }
       if (info.info.dir != directoryOrig) {
         isExternal = true;

lucene/core/src/test/org/apache/lucene/index/TestConcurrentMergeScheduler.java

@@ -454,7 +454,7 @@ public class TestConcurrentMergeScheduler extends LuceneTestCase {
             // return many merges.
             MergeSpecification spec = new MergeSpecification();
             List<SegmentCommitInfo> oneMerge = new ArrayList<>();
-            for (SegmentCommitInfo sci : segmentsToMerge.keySet()) {
+            for (SegmentCommitInfo sci : segmentInfos) {
               oneMerge.add(sci);
               if (oneMerge.size() >= 10) {
                 spec.add(new OneMerge(new ArrayList<>(oneMerge)));