appview: add nil-check before resubmit-check

resubmit checks are valid only for users that are logged in

appview/state/pull.go

···
 
 	mergeCheckResponse := s.mergeCheck(f, pull)
 	resubmitResult := pages.Unknown
-	if user.Did == pull.OwnerDid {
+	if user != nil && user.Did == pull.OwnerDid {
 		resubmitResult = s.resubmitCheck(f, pull)
 	}