archive/zip: remove an allocation, speed up a test

Update #6138

TestOver65kFiles spends all its time garbage collecting.
Removing the 1.4 MB of allocations per each of the 65k
files brings this from 34 seconds to 0.23 seconds.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12894043
2 files changed