src: cleanup uv_fs_t regardless of success or not

[legendecas]

1. Cleanup uv_fs_t regardless of success or not.
2. Prevent from accessing members of uv_fs_t after cleanup.

This is a quite recent change, so I'm requesting original author @joyeecheung 's review.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/38562/

• not ok 2626 parallel/test-worker-cleanexit-with-moduleload

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/38569/

• ld terminated with signal 9

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/38573/

[jasnell]

An alternative approach (that may be a bit nicer here) is to use a smart-pointer type of mechanism like we use elsewhere... something like...

struct UvFsReq {
  uv_fs_t req;
  ~UvFsReq() {
    uv_fs_req_cleanup(&req);
  }
}

[legendecas]

Sounds great, I'll do an update.

[legendecas]

After several tries, I found that wrapping with a naive data struct did not work well in the case. We have to access members of uv_fs_t and passing the pointer of uv_fs_t back to uv, like:

uv_fs_t req;
uv_fs_open(nullptr, &req, path, O_RDONLY, 0, nullptr);  // <- either we write with `&UvFsReq.req` or using operators to automatically convert (which might not be very straightforward since it implicit converting from data type to pointer type)
if (req.result < 0); // <- accessing member of `uv_fs_t`, either we write with UvFsReq.req.result or explicitly declare the member proxy in UvFsReq like `UvFsReq.result()`. 

This is too much for the intent, a OnScopeLeave just fit in well.

As such, I'd believe it is more readable and straightforward to use uv_fs_t here. And there are a lot of precedents in the code base.

[legendecas]

Landed in e4eadb2