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

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


Reviewed-By: Sam <Roberts &lt;vieuxtech@gmail.com>
Reviewed-By: Matteo <Collina &lt;matteo.collina@gmail.com>
Reviewed-By: Ben <Noordhuis &lt;info@bnoordhuis.nl>
Reviewed-By: Rich <Trott &lt;rtrott@gmail.com>
Reviewed-By: Anna <Henningsen &lt;anna@addaleax.net>
Reviewed-By: Shelley <Vohr &lt;codebytere@gmail.com>
Reviewed-By: Tobias <Nießen &lt;tniessen@tnie.de>
Reviewed-By: Gireesh <Punathil &lt;gpunathi@in.ibm.com>
Reviewed-By: Beth <Griggs &lt;Bethany.Griggs@uk.ibm.com>
Reviewed-By: Michael <Dawson &lt;michael_dawson@ca.ibm.com>