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

Update Issue 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://codereview.appspot.com/12894043