src: don't overwrite non-writable vm globals

Check that the property doesn't have the read-only flag set before
overwriting it.

Fixes: https://github.com/nodejs/node/issues/10223
PR-URL: https://github.com/nodejs/node/pull/10227


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>