Avoid failing if we can't acquire the searcher

Author: carlosdelest

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -2849,8 +2849,11 @@ public synchronized void beforeMerge(OnGoingMerge merge) {
 
         @Override
         protected long estimateMergeMemory(MergePolicy.OneMerge merge) {
-            try (Searcher searcher = acquireSearcher("merge_memory_estimation")) {
+            try (Searcher searcher = acquireSearcher("merge_memory_estimation", SearcherScope.INTERNAL)) {
                 return SegmentMergeMemoryEstimator.estimateSegmentMemory(merge, searcher.getIndexReader());
+            } catch (AlreadyClosedException e) {
+                failOnTragicEvent(e);
+                return 0L;
             }
         }