Fix double login issue

- Removed session existence check in killme
- Changed returned value in session_write for empty data

boot.php

@@ -928,14 +928,8 @@ function login($register = false, $hiddens = false)
  */
 function killme()
 {
-	global $session_exists;
-
 	if (!get_app()->is_backend()) {
-		if (!$session_exists) {
-			session_abort();
-		} else {
-			session_write_close();
-		}
+		session_write_close();
 	}
 
 	exit();

include/session.php

@@ -61,10 +61,14 @@ function ref_session_write($id, $data)
 {
 	global $session_exists, $session_expire;
 
-	if (!$id || !$data) {
+	if (!$id) {
 		return false;
 	}
 
+	if (!$data) {
+		return true;
+	}
+
 	$expire = time() + $session_expire;
 	$default_expire = time() + 300;