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>