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 #7069

R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/48150043

src/pkg/archive/zip/reader.go

@@ -253,7 +253,7 @@ func readDirectoryHeader(f *File, r io.Reader) error {
 			}
 			if tag == zip64ExtraId {
 				// update directory values from the zip64 extra block
-				eb := readBuf(b)
+				eb := readBuf(b[:size])
 				if len(eb) >= 8 {
 					f.UncompressedSize64 = eb.uint64()
 				}