x509:reject certificates with a zero serial number (RFC 5280)

[dwivedys]

Description

Fixes #10763

Per RFC 5280 (and Errata #3200), a certificate's serial number
MUST be a positive, non-zero integer. This adds a check in
mbedtls_x509_crt_parse_der_core() (library/x509_crt.c) that rejects a certificate
whose serial number is entirely zero bytes, returning
MBEDTLS_ERR_X509_INVALID_SERIAL.

A new test case is added in test_suite_x509parse.data covering
this.

Note: this change causes ~12 pre-existing tests to fail, because
several existing test certificates in the test suite were
generated with a serial number of zero and were previously
accepted.

I have not modified those test fixtures.
Flagging this for maintainer input — happy to regenerate the
affected test certs in a follow-up commit if that's the
preferred approach.

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[bensze01]

Thank you for your contribution.

To quote RFC 5280

Note: Non-conforming CAs may issue certificates with serial numbers
that are negative or zero. Certificate users SHOULD be prepared to
gracefully handle such certificates.

As such, I don't think it's useful to reject zero-serial certificates.

[dwivedys]

Sorry since this was my first time working on an open source project may be I am not fully conversant with how you all handle these things. What I do want to understand is that I worked on this basis my understanding of the ask that zero certs need to be rejected so I did it. Now you are saying its not useful to do so and they should be handled gracefully. What does this mean? Meaning we wont reject them but perhaps send a useful message back to the user saying the cert is invalid which to me is tantamount to rejecting it?