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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines