Play it a little safer when reassigning login uri

src/signout.php

@@ -38,7 +38,7 @@ if (! sqgetGlobalVar('base_uri', $base_uri, SQ_SESSION) ) {
 $login_uri = 'login.php';
 
 $hook_results = do_hook('logout', $login_uri);
-$login_uri = $hook_results[1];
+if (!empty($hook_results[1])) $login_uri = $hook_results[1];
 
 sqsession_destroy();