Fix mbstring check in update.php

Fix mbstring check in update.php

Maintenance::error() will not cause the script to exit unless the
$die argument, when cast as an integer, is greater than 0. Since
the default value is 0, specify a different one, as in the other
calls from within the same function.

Follows-up 943563062f0a6995.

Change-Id: I3af72b1cd87c63d7f57436866633e86c272e904f