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

@@ -387,15 +387,15 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 	if rt, ok := dst.(ReaderFrom); ok {
 		return rt.ReadFrom(src)
 	}
-	size := 32 * 1024
-	if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
-		if l.N < 1 {
-			size = 1
-		} else {
-			size = int(l.N)
-		}
-	}
 	if buf == nil {
+		size := 32 * 1024
+		if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
+			if l.N < 1 {
+				size = 1
+			} else {
+				size = int(l.N)
+			}
+		}
 		buf = make([]byte, size)
 	}
 	for {