fix(napi): do not call release tsfn in Drop when ref count is 0

[Brooooooklyn]

Before fix:

napi4 › threadsafe_function.ts › tsfn dua instance

  test_module/__test__/napi4/threadsafe_function.spec.ts:95

   94:   }                                                            
   95:   t.notThrows(() => {                                          
   96:     execSync(`node ${join(__dirname, 'tsfn-dua-instance.js')}`)

  Function threw:

  Error {
    output: [
      null,
      Buffer @Uint8Array [],
      Buffer @Uint8Array [],
    ],
    pid: 26327,
    signal: 'SIGABRT',
    status: null,
    stderr: Buffer @Uint8Array [],
    stdout: Buffer @Uint8Array [],
    message: 'Command failed: node /Users/longyinan/workspace/github/napi-rs/test_module/__test__/napi4/tsfn-dua-instance.js',
  }

  › test_module/__test__/napi4/threadsafe_function.spec.ts:96:5
  › test_module/__test__/napi4/threadsafe_function.spec.ts:95:5

  ─

  1 test failed

Close #518

[williamluke4]

Thanks @Brooooooklyn