Error adding data segments

[chenyan2002]

Describe the Bug

Adding a new data segment produces bad Wasm binary.

Steps to Reproduce

    let mut m = Module::with_config(ModuleConfig::new());
    m.data.add(DataKind::Passive, vec![1,2,3,4]);
    m.emit_wasm_file("out.wasm").unwrap();

Running wasm2wat out.wasm -o out.wat gets the following error

000000e: error: invalid section code: 12

Expected Behavior

wasm2wat should produce no error.