crypto/md5: uniform Write func

Unification of implementation of existing md5.Write function
with other implementations (sha1, sha256, sha512).

Change-Id: I58ae02d165b17fc221953a5b4b986048b46c0508
Reviewed-on: https://go-review.googlesource.com/16621Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/md5/md5.go

@@ -62,16 +62,10 @@ func (d *digest) Write(p []byte) (nn int, err error) {
 	nn = len(p)
 	d.len += uint64(nn)
 	if d.nx > 0 {
-		n := len(p)
-		if n > chunk-d.nx {
-			n = chunk - d.nx
-		}
-		for i := 0; i < n; i++ {
-			d.x[d.nx+i] = p[i]
-		}
+		n := copy(d.x[d.nx:], p)
 		d.nx += n
 		if d.nx == chunk {
-			block(d, d.x[0:chunk])
+			block(d, d.x[:])
 			d.nx = 0
 		}
 		p = p[n:]