Quick fix for Kerberos: don't use hardcoded userPlusRealm length

Hey Gustavo,

Looks like I made a silly mistake in my SSPI code, forgot to take out the hardcoded username length.
Sure, the process of re-allocating and re-copying the data looks suspect, but I'll leave that up to you.

That said, let's please fix the Go side of this. That buffersToFree list is completely unnecessary, as suggested in the comment above, and it's also going to crash the application if there are errors, since the buffer is not allocated. It should be trivial to free with defer as suggested.