database: report connection failure in _mysql_connect()

This is used in the upgrade script and some old web pages.
You need to check $mysql->connect_errno after doing new mysqli

html/inc/db.inc

@@ -48,6 +48,9 @@ if (MYSQLI) {
             $port = null;
         }
         $mysqli = @new mysqli($host, $user, $pass, $dbname, $port);
+        if ($mysqli->connect_errno) {
+            return null;
+        }
         return $mysqli;
     }
     function _mysql_query($q) {