html/template: 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: I6f7a8609f2453f622a1fa94a50c99d2e04d5fbcd
GitHub-Last-Rev: 3a5d9b1e9e202327af17cc1b93bfa69f6701af84
GitHub-Pull-Request: golang/go#30215
Reviewed-on: https://go-review.googlesource.com/c/162477Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/html/template/escape_test.go

@@ -1869,8 +1869,7 @@ func TestErrorOnUndefined(t *testing.T) {
 	err := tmpl.Execute(nil, nil)
 	if err == nil {
 		t.Error("expected error")
-	}
-	if !strings.Contains(err.Error(), "incomplete") {
+	} else if !strings.Contains(err.Error(), "incomplete") {
 		t.Errorf("expected error about incomplete template; got %s", err)
 	}
 }