The problem here is that binary input is valid according to the spec [1]. It's not malicious input in the sense that a programmer is using an interface deliberately wrong but rather in the sense that a counterparty could send you a non-garbage certificate that contains that data - which would be valid, but still break this code. That's not comparable to passing a bad pointer or a bad length.

[1] at least according to the post. The fact that gnutls added a binary interface later seems to support that reading.