Fix user router possbile panic (#29751) (#29786)

regression from #28023
backport #29751

tests/integration/user_test.go

@@ -243,6 +243,8 @@ func testExportUserGPGKeys(t *testing.T, user, expected string) {
 }
 
 func TestGetUserRss(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
 	user34 := "the_34-user.with.all.allowedChars"
 	req := NewRequestf(t, "GET", "/%s.rss", user34)
 	resp := MakeRequest(t, req, http.StatusOK)
@@ -253,6 +255,13 @@ 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)
 	}
+
+	req = NewRequestf(t, "GET", "/non-existent-user.rss")
+	MakeRequest(t, req, http.StatusNotFound)
+
+	session := loginUser(t, "user2")
+	req = NewRequestf(t, "GET", "/non-existent-user.rss")
+	session.MakeRequest(t, req, http.StatusNotFound)
 }
 
 func TestListStopWatches(t *testing.T) {