Fix PKCS7 parser failing to parse degenerated certificate messages

[stevendpclark]

A properly formatted PKCS7 message with unsigned multiple certificates is hitting a conditional check that the offset value is greater than the buffer. While that is true, in this use case we don't attempt to read from the buffer again so the check for buffer boundaries causes the parser to fail to read a message.

Tweak the offset checks to occur just before we are about to read from buf using the offset variable and not after we increment the offset variable. Augment the TestDegenerateCertificate to parse its own generated message along with OpenSSL that exposes the issue before the fixes to ber.go

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[fairclothjm]

LGTM, thanks!

[victorr]

👍