Fix error 1105 while changing bug status

Attempting to change the bug status from bug_change_status_page.php
(for example, acknowledging or confirming a bug) results in APPLICATION
ERROR 1105.

'last_updated' is an int in $t_existing_bug, but is read as string into
$t_updated_bug, which results in a failed exact compare (!==) at line
116.

Thanks to Alex Volkov for the research and proposed fix.

Fixes 0020075