net/textproto: prevent test from failing with nil pointer dereference

The variable err could have nil value when we call err.Error(),
because after we check it for nil above we continue the test
(t.Errorf doesn't stop the test execution).

Updates #30208

Change-Id: Ibcf38698326c69c06068989510311e37806995c6
GitHub-Last-Rev: 3ab20f6d7fe34ed9b777e0894b57166d173de8ca
GitHub-Pull-Request: golang/go#30214
Reviewed-on: https://go-review.googlesource.com/c/162457Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/textproto/reader_test.go

@@ -332,7 +332,7 @@ func TestReadMultiLineError(t *testing.T) {
 	if msg != wantMsg {
 		t.Errorf("ReadResponse: msg=%q, want %q", msg, wantMsg)
 	}
-	if err.Error() != "550 "+wantMsg {
+	if err != nil && err.Error() != "550 "+wantMsg {
 		t.Errorf("ReadResponse: error=%q, want %q", err.Error(), "550 "+wantMsg)
 	}
 }