net: fix a nit found by vet

Change-Id: Ic965b48bdc0e0ffba9bed5ff6b3d28ff742f8b2d
Reviewed-on: https://go-review.googlesource.com/9100Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/ipsock_test.go

@@ -226,7 +226,7 @@ func TestAddrList(t *testing.T) {
 		}
 		if tt.err != nil {
 			if len(addrs) != 0 {
-				t.Errorf("#%v: got %v; want 0", len(addrs))
+				t.Errorf("#%v: got %v; want 0", i, len(addrs))
 			}
 			continue
 		}