Go implementation Buffer.GetBytes panics

[toqueteos]

I'm digging into Go's codebase and found a bug in Buffer.GetBytes.

Here's a playpen to see it in action:
https://play.golang.org/p/3-1uLvXD83

I'll send a PR with a fix for this and the corresponding test.

[wirepair]

woops yeah, probably want to check the Pos + length.. my bad, thanks for finding this!