Only try to read new segments info if we really flushed the index (#20474)

There is no reason to read the current segments info unless we flushed
/ committed the lucene index.

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

@@ -789,7 +789,6 @@ public class InternalEngine extends Engine {
                     } catch (Exception e) {
                         throw new FlushFailedEngineException(shardId, e);
                     }
-                }
                     /*
                      * we have to inc-ref the store here since if the engine is closed by a tragic event
                      * we don't acquire the write lock and wait until we have exclusive access. This might also
@@ -814,6 +813,7 @@ public class InternalEngine extends Engine {
                     } finally {
                         store.decRef();
                     }
+                }
                 newCommitId = lastCommittedSegmentInfos.getId();
             } catch (FlushFailedEngineException ex) {
                 maybeFailEngine("flush", ex);