PKCS7 parser accepts invalid messages

[DemiMarie]

Summary

I found various bugs in the PKCS7 parser. These are not the memory safety problems found by OSS-Fuzz, but rather cases where the code did not conform to the standard or had an API that was virtually unusable.

System information

Mbed TLS version (number or commit id): 590ae53
Operating system and version: All
Configuration (if not default, please attach mbedtls_config.h): Default
Compiler and options (if you used a pre-built binary, please indicate how you obtained it): N/A
Additional environment information: This was found by manual source review

Expected behavior

PKCS7 parser rejects invald messages and provides a way to obtain the signed data.

Actual behavior

PKCS7 parser accepts invald messages and does not provide a way to obtain the signed data.

Steps to reproduce

Review the source code.

Additional information

I have a patch.