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

Update #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://golang.org/cl/12894043

src/pkg/archive/zip/reader.go

@@ -179,9 +179,8 @@ func (r *checksumReader) Close() error { return r.rc.Close() }
 // findBodyOffset does the minimum work to verify the file has a header
 // and returns the file body offset.
 func (f *File) findBodyOffset() (int64, error) {
-	r := io.NewSectionReader(f.zipr, f.headerOffset, f.zipsize-f.headerOffset)
 	var buf [fileHeaderLen]byte
-	if _, err := io.ReadFull(r, buf[:]); err != nil {
+	if _, err := f.zipr.ReadAt(buf[:], f.headerOffset); err != nil {
 		return 0, err
 	}
 	b := readBuf(buf[:])

src/pkg/archive/zip/zip_test.go

@@ -17,14 +17,14 @@ import (
 )
 
 func TestOver65kFiles(t *testing.T) {
-	if testing.Short() {
-		t.Skip("slow test; skipping")
-	}
 	buf := new(bytes.Buffer)
 	w := NewWriter(buf)
 	const nFiles = (1 << 16) + 42
 	for i := 0; i < nFiles; i++ {
-		_, err := w.Create(fmt.Sprintf("%d.dat", i))
+		_, err := w.CreateHeader(&FileHeader{
+			Name:   fmt.Sprintf("%d.dat", i),
+			Method: Store, // avoid Issue 6136 and Issue 6138
+		})
 		if err != nil {
 			t.Fatalf("creating file %d: %v", i, err)
 		}