src: fix vm module for strict mode

This patch fixes the problem with variables that are declared only on the sandbox but not on the global proxy.

Fixes: https://github.com/nodejs/node/issues/12300

Checklist

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

Affected core subsystem(s)

src