NullPointerException in FsTranslog when reverting transient log

This happens when reverting the trans transaction log on failure, and when that happens, actually we might have failed on the transient translog creation to being with....
fixes #4223

src/main/java/org/elasticsearch/index/translog/fs/FsTranslog.java

@@ -291,15 +291,19 @@ public class FsTranslog extends AbstractIndexShardComponent implements Translog
 
     @Override
     public void revertTransient() {
-        FsTranslogFile old;
+        FsTranslogFile tmpTransient;
         rwl.writeLock().lock();
         try {
-            old = trans;
+            tmpTransient = trans;
             this.trans = null;
         } finally {
             rwl.writeLock().unlock();
         }
-        old.close(true);
+        // previous transient might be null because it was failed on its creation
+        // for example
+        if (tmpTransient != null) {
+            tmpTransient.close(true);
+        }
     }
 
     public byte[] read(Location location) {