[GITEA] Fix NPE in UsernameSubRoute

- When the user is not found in `reloadparam`, early return when the
user is not found to avoid calling `IsUserVisibleToViewer` which in turn
avoids causing a NPE.
- This fixes the case that a 500 error and 404 error is shown on the
same page.
- Add integration test for non-existant user RSS.
- Regression by c6366089df

(cherry picked from commit f0e0696278)

routers/web/user/home.go

@@ -710,12 +710,15 @@ func UsernameSubRoute(ctx *context.Context) {
 	reloadParam := func(suffix string) (success bool) {
 		ctx.SetParams("username", strings.TrimSuffix(username, suffix))
 		context_service.UserAssignmentWeb()(ctx)
+		if ctx.Written() {
+			return false
+		}
 		// check view permissions
 		if !user_model.IsUserVisibleToViewer(ctx, ctx.ContextUser, ctx.Doer) {
 			ctx.NotFound("user", fmt.Errorf(ctx.ContextUser.Name))
 			return false
 		}
-		return !ctx.Written()
+		return true
 	}
 	switch {
 	case strings.HasSuffix(username, ".png"):

tests/integration/user_test.go

@@ -243,6 +243,9 @@ func testExportUserGPGKeys(t *testing.T, user, expected string) {
 }
 
 func TestGetUserRss(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	t.Run("Normal", func(t *testing.T) {
 		user34 := "the_34-user.with.all.allowedChars"
 		req := NewRequestf(t, "GET", "/%s.rss", user34)
 		resp := MakeRequest(t, req, http.StatusOK)
@@ -253,6 +256,12 @@ func TestGetUserRss(t *testing.T) {
 			description, _ := rssDoc.ChildrenFiltered("description").Html()
 			assert.EqualValues(t, "<p dir="auto">some <a href="https://commonmark.org/" rel="nofollow">commonmark</a>!</p>\n", description)
 		}
+	})
+	t.Run("Non-existent user", func(t *testing.T) {
+		session := loginUser(t, "user2")
+		req := NewRequestf(t, "GET", "/non-existent-user.rss")
+		session.MakeRequest(t, req, http.StatusNotFound)
+	})
 }
 
 func TestListStopWatches(t *testing.T) {