io: simplified a small part of copyBuffer

Change-Id: I0b7052103174f0864ee9714f76f8f78f2a988777
Reviewed-on: https://go-review.googlesource.com/30719Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/io/io.go

@@ -402,11 +402,10 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 				break
 			}
 		}
-		if er == EOF {
-			break
-		}
 		if er != nil {
-			err = er
+			if er != EOF {
+				err = er
+			}
 			break
 		}
 	}