acquireSearcher should fail with state failure when not set/closed

also fixes #4232

src/main/java/org/elasticsearch/index/engine/robin/RobinEngine.java

@@ -684,6 +684,9 @@ public class RobinEngine extends AbstractIndexShardComponent implements Engine {
     @Override
     public final Searcher acquireSearcher(String source) throws EngineException {
         SearcherManager manager = this.searcherManager;
+        if (manager == null) {
+            throw new EngineClosedException(shardId);
+        }
         try {
             IndexSearcher searcher = manager.acquire();
             return newSearcher(source, searcher, manager);