MM-29812: Fix a panic in auth code if GetChannel returns an error. (#16010)

* MM-29812: Don't panic if GetChannel returns an error in auth code.

* Fix gofmt

* Add further mocking.

* Another mock.

* More mocks.

app/authorization.go

@@ -80,7 +80,7 @@ func (a *App) SessionHasPermissionToChannel(session model.Session, channelId str
 		return true
 	}
 
-	if channel.TeamId != "" {
+	if appErr == nil && channel.TeamId != "" {
 		return a.SessionHasPermissionToTeam(session, channel.TeamId, permission)
 	}