archive/tar: use built-in clear to simplify code

Change-Id: I0e55dd68d92c39aba511b55368bf50d929d75f86
GitHub-Last-Rev: 1743014
GitHub-Pull-Request: #66158
Reviewed-on: https://go-review.googlesource.com/c/go/+/569696
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: qiulaidongfeng <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: apocelipes

src/archive/tar/reader.go

@@ -811,9 +811,7 @@ func (sr sparseFileReader) physicalRemaining() int64 {
 type zeroReader struct{}
 
 func (zeroReader) Read(b []byte) (int, error) {
-	for i := range b {
-		b[i] = 0
-	}
+	clear(b)
 	return len(b), nil
 }