async_wrap: add a missing case to test-async-wrap-throw-no-init

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

async_wrap, test

Description of change

This covers this line in "src/async-wrap.cc". /R= @trevnorris