os,vm: fix segfaults and CHECK failure

Fixes multiple possible segmentation faults in node_contextify.cc and
an assertion failure in node_os.cc

Fixes: https://github.com/nodejs/node/issues/12369
Fixes: https://github.com/nodejs/node/issues/12370
PR-URL: https://github.com/nodejs/node/pull/12371


Reviewed-By: Anna Henningsen <anna@addaleax.net>