GetFeeds must always discard actions with dangling repo_id (#19598)

* GetFeeds must always discard actions with dangling repo_id

See https://discourse.gitea.io/t/blank-page-after-login/5051/12
for a panic in 1.16.6.

* add comment to explain the dangling ID in the fixture

* loadRepoOwner must not attempt to use a nil action.Repo

* make fmt

Co-authored-by: Loïc Dachary <loic@dachary.org>

models/fixtures/action.yml

@@ -56,3 +56,11 @@
   repo_id: 8 # public
   is_private: false
   created_unix: 1603011540 # grouped with id:7
+
+- id: 8
+  user_id: 1
+  op_type: 12 # close issue
+  act_user_id: 1
+  repo_id: 1700 # dangling intentional
+  is_private: false
+  created_unix: 1603011541