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.

PR-URL: https://github.com/nodejs/node/pull/16487
Fixes: https://github.com/nodejs/node/issues/12300


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>