Fix WriteBatch::iterate memory leak (#658)

Fix https://github.com/tikv/rust-rocksdb/issues/654Signed-off-by: yuqi1129 <yuqi4733@gmail.com>

src/write_batch.rs

@@ -124,6 +124,8 @@ impl WriteBatch {
                 delete_fn,
                 delete_cf_fn,
             );
+            // Let rust free the memory
+            let _ = Box::from_raw(state as *mut WriteBatchCallback);
         }
     }