check write object to be non null before calling close on it

Author: Vladimir Kotal

src/org/opensolaris/opengrok/history/FileHistoryCache.java

@@ -592,7 +592,9 @@ private void storeLatestCachedRevision(Repository repository, String rev) {
                 ex.getCause());
         } finally {
            try {
-               writer.close();
+               if (writer != null) {
+                   writer.close();
+               }
            } catch (IOException ex) {
                logger.log(Level.INFO, "cannot close file: " + ex);
            }