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.
There seems to be a leak of this allocation. I suggest dropping the memcpy below, and instead simply doing:

    *buffer = msg;
    *buffer_length = ...

so that you pass the reference back into the Go code rather than losing it.

Then, the Go code that calls this also seems a bit problematic. It should probably do something alone the lines of:

```
if buffer != C.PVOID(nil) {
        defer C.free(unsafe.Pointer(buffer))
}
```

This will avoid the need for buffersToFree, and most importantly will not crash the whole process when there are any errors in send_client_authz_id that prevent it from sending a proper buffer. Note the defer.. this allows handling the deallocation early on, and then forgetting about it in a good sense.

Note that GoBytes will copy the data, so it is okay to drop the underlying data.