io: calculate buffer size only when needed

Change-Id: I930be9027fb972198b3d44816a5e4f53ff7eb5ea
Reviewed-on: https://go-review.googlesource.com/111642Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/io/io.go

@@ -387,6 +387,7 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 	if rt, ok := dst.(ReaderFrom); ok {
 		return rt.ReadFrom(src)
 	}
+	if buf == nil {
 		size := 32 * 1024
 		if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
 			if l.N < 1 {
@@ -395,7 +396,6 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 				size = int(l.N)
 			}
 		}
-	if buf == nil {
 		buf = make([]byte, size)
 	}
 	for {