fix: do not crash when using a closed fs event watcher

[codebytere]

Resolveselectron/electron#14844.

This PR backports nodejs/node#20985 to 3-0-x.

The original fix went out in v10.5.0, so we don't need to backport it anywhere else since we'll cut 4-0-x with at least Node v10.10.0.

/cc @pfrazee

[ryzokuken]

Looks great!

[bpasero]

@codebytere hey, you might want to look at microsoft/vscode#70566 (comment) where a user in VSCode sees that crash in the picture. Could it be that checking for null is not good because it could be undefined too? Any reason for this change and divergence from the node.js original implementation?

[bpasero]

Ah ok, looks like this came in as a backport. Maybe Electron should follow up and check if a more recent patch is available that avoids this issue.

[bpasero]

I am actually clueless how the crash in microsoft/vscode#70566 (comment) can happen. Looking at the code it seems like this._handle is either null or has its value ❓ .