[v14.x] worker: do not crash when JSTransferable lists untransferable value
This can currently be triggered when posting a closing FileHandle.
Refs: https://github.com/nodejs/node/pull/34746#issuecomment-673675333
PR-URL: https://github.com/nodejs/node/pull/34766 Reviewed-By: Richard Lau riclau@uk.ibm.com Reviewed-By: David Carlier devnexen@gmail.com Reviewed-By: James M Snell jasnell@gmail.com