home: don't crash in branch_tag_switcher when an invalid repo name is given

rhodecode/controllers/home.py

@@ -81,7 +81,7 b' class HomeController(BaseController):'
     def branch_tag_switcher(self, repo_name):
         if request.is_xhr:
             c.rhodecode_db_repo = Repository.get_by_repo_name(c.repo_name)
-            c.rhodecode_repo = c.rhodecode_db_repo.scm_instance
-            return render('/switch_to_list.html')
-        else:
-            raise HTTPBadRequest()
+            if c.rhodecode_db_repo:
+                c.rhodecode_repo = c.rhodecode_db_repo.scm_instance
+                return render('/switch_to_list.html')
+        raise HTTPBadRequest()