n-api: fix coverity scan report

Coverity was reporting _request.work_req as not being initialized.
Add memset to ensure all of _request is initialized.

PR-URL: https://github.com/nodejs/node/pull/12365
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node_api.cc

@@ -2664,6 +2664,7 @@ class Work {
     _data(data),
     _execute(execute),
     _complete(complete) {
+    memset(&_request, 0, sizeof(_request));
     _request.data = this;
   }