crypto: fix zero byte allocation assertion failure

When an empty string was passed, malloc might have returned a nullptr
depending on the platform, causing an assertion failure. This change
makes private key parsing behave as public key parsing does, causing
a BIO error instead that can be caught in JS.

Fixes: https://github.com/nodejs/node/issues/25247

PR-URL: https://github.com/nodejs/node/pull/25248


Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>