crypto: fix assertion caused by unsupported ext

`X509V3_EXT_print` can return value different from `1` if the X509
extension does not support printing to a buffer. Instead of failing with
an unrecoverable assertion - replace the relevant value in the hashmap
with a JS null value.

Fixes: https://hackerone.com/reports/746733

Backport-PR-URL: https://github.com/nodejs-private/node-private/pull/193
PR-URL: https://github.com/nodejs-private/node-private/pull/175


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>