fix: prevent 500 message on invalid username (#8236)

Bug introduced by #7998

### Tests

- go to your settings page https://v12.next.forgejo.org/user/settings
- try to change to an invalid username (like `.well--invalid`)
- verify that no 500 error is shown, only a flash message

I tried to add a test in `tests/integration/user_test.go`, but failed to catch the error message:

```
		resp := session.MakeRequest(t, req, http.StatusOK)
		txt := resp.Body.String()
		t.Log(txt) // no template error??
		t.FailNow()
```

### Release notes

- [x] I do not want this change to show in the release notes.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8236
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

routers/web/user/setting/profile.go

@@ -66,6 +66,9 @@ func ProfilePost(ctx *context.Context) {
 	ctx.Data["DisableGravatar"] = setting.Config().Picture.DisableGravatar.Value(ctx)
 	ctx.Data["CooldownPeriod"] = setting.Service.UsernameCooldownPeriod
 	ctx.Data["CommonPronouns"] = commonPronouns
+	ctx.Data["MaxAvatarFileSize"] = setting.Avatar.MaxFileSize
+	ctx.Data["MaxAvatarWidth"] = setting.Avatar.MaxWidth
+	ctx.Data["MaxAvatarHeight"] = setting.Avatar.MaxHeight
 
 	if ctx.HasError() {
 		ctx.HTML(http.StatusOK, tplSettingsProfile)