The buffer overflow is in the C version of the algorithm (and likely related to loop condition checks idiomatic to C's for-loops). The Go version is a fresh implementation, not a wrapping of the C version. I'm no Go programmer, but if I'm not mistaken, the Go implementation just eats little slices of the input buffer until no more buffer is left, leaving all the overflow-danger to the Go array implementation:

https://github.com/golang/crypto/blob/642fcc37f5043eadb2509c...

Possibly not as fast as C, but easier to reason about, I'd say.