archive/zip: fix bug reading zip64 files
archive/zip: fix bug reading zip64 files

ZIP64 Extra records are variably sized, but we weren't capping
our reading of the extra fields at its previously-declared
size.

No test because I don't know how to easily create such files
and don't feel like manually construction one.  But all
existing tests pass, and this is "obviously correct" (queue
laughter).

Fixes Issue 7069

R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/48150043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang