prevent a user to edit someone else's talk

the bug was introduced by b8f2db2
and caused by the unnecessary complexity of the test,
which has nested unless with complicated conditions

this patch only fixes the bug, not the unnecessary complexity of the test

lib/Act/Handler/Talk/Edit.pm

@@ -79,7 +79,7 @@ sub handler {
     unless ($Request{user}->is_talks_admin) {
         unless ( ($talk && $talk->user_id == $Request{user}->user_id
                         && ($Config->talks_edition_open || $Config->talks_submissions_open))
-                || $Config->talks_submissions_open )
+                || (!$talk && $Config->talks_submissions_open ))
         {
             $Request{status} = NOT_FOUND;
             return;