language: fix performance bugs

- Match should not allocate
- Prevent allocation by not including a string in an invalid Extension.

Change-Id: Icfe091121d99945b70084214ae9e76c79d6ec5b8
Reviewed-on: https://go-review.googlesource.com/30271
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed