Null Pointer Dereference in AddTime API Causes Authenticated Denial of Service

Affected: Go: code.gitea.io/gitea

### Summary

The AddTime API handler continues execution after an error returned by `GetUserByName()`.

When a repository administrator specifies a non-existent user name, an error response is generated but execution does not stop. Subsequent code dereferences a nil user pointer, resulting in a runtime panic.

### Details

Affected endpoint:

```http POST /api/v1/repos/{owner}/{repo}/issues/{index}/times ```

Affected file:

```text routers/api/v1/repo/issue_tracked_time.go ```

Relevant code:

```go user, err = user_model.GetUserByName(ctx, form.User) if err != nil { ctx.APIErrorInternal(err) // missing return } ```

Execution continues to:

```go trackedTime, err := issues_model.AddTime( ctx, user, issue, form.Time, created, ) ```

When `GetUserByName()` fails, `user` is nil.

The subsequent call dereferences the nil pointer and triggers a runtime panic.

### Proof of Concept

Using a repository administrator account:

```http POST /api/v1/repos/owner/repo/issues/1/times Content-Type: application/json

{ "time": 3600, "user_name": "nonexistent_user_xyz" } ```

Result:

```text HTTP 500 runtime error: invalid memory address or nil pointer dereference ```

The stack trace indicates execution reaches the AddTime code path with a nil user object.

### Impact

An authenticated repository administrator can repeatedly trigger server-side panics through the affected endpoint.

Depending on deployment configuration and panic recovery behavior, this m…

Ein Angreifer kann mehrere Schwachstellen in Gitea ausnutzen, um beliebigen Programmcode auszuführen, erweiterte Berechtigungen zu erlangen, Sicherheitsmaßnahmen zu umgehen, Daten zu manipulieren, sensible Informationen offenzulegen, Sitzungen zu übernehmen oder wiederzuverwenden oder einen Denial-of-Service-Zustand auszulösen.