Synchronize isClosed.

src/main/java/com/jakewharton/disklrucache/DiskLruCache.java

@@ -605,7 +605,7 @@ public synchronized boolean remove(String key) throws IOException {
   }
 
   /** Returns true if this cache has been closed. */
-  public boolean isClosed() {
+  public synchronized boolean isClosed() {
     return journalWriter == null;
   }