storage_backend_fs: avoid NULL dereference on opendir failure

* src/storage_backend_fs.c (virStorageBackendFileSystemRefresh):
Don't call closedir on a NULL pointer.

src/storage_backend_fs.c

@@ -983,7 +983,8 @@ no_memory:
     /* fallthrough */
 
  cleanup:
-    closedir(dir);
+    if (dir)
+        closedir(dir);
     virStorageVolDefFree(vol);
     virStoragePoolObjClearVols(pool);
     return -1;