X.509: SubjectAlternativeName components are not parsed

[hanno-becker]

Context: This is about the parsing of the SubjectAlternativeNames extension within an X.509 certificate. There are differently structured types of alternative name components, and while we don't store them permanently, their structural sanity is checked by mbedtls_x509_parse_subject_alt_name() checked as part of mbedtls_x509_crt_parse() and friends.

Issue: mbedtls_x509_parse_subject_alt_name() is called here:

https://github.com/ARMmbed/mbedtls-restricted/blob/62be28b525076a022b31b0cff8f0e296f9b0bdc6/library/x509_crt.c#L645-L670

It can be seen that tag hasn't yet been stored in cur->buf->tag, so that mbedtls_x509_parse_subject_alt_name() cannot recognize the specific name and will always return MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE, which is silently ignored afterwards.