fix modifying existing zipfile with zip64 enabled

The local header size computed from the central directory entry
is incorrect due to the Zip64Placeholder in the local entry.
This caused us to seek to the wrong location when copying an
unchanged compressed data stream.

(The same problem could occur when modifying any zip file where
the local header and central directory header contain different
variable-sized fields, so it's a good idea not to trust the CD
to tell us the local header size in any case.)