[ticket/11829] Use report_closed to determine status in MCP report_details

http://tracker.phpbb.com/browse/PHPBB3-11829

Instead of using post_reported of the post or message_reported of the pm, use
report_closed of the report itself to reliably determine whether this
particular report is closed or not in the report_details view of the MCP.
This fixes a bug where the report_details view would not show that the report
shown was closed and display a "Close report" button that had no effect.