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

webfd/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc