Fix warning on Special:ListGroupRights for group with invalid group page

Avoids
Notice: Linker::link: Requires $target to be a Title object. [Called
from SpecialListGroupRights::execute in
\includes\specials\SpecialListgrouprights.php at line 97]

Change-Id: Ie90ea33b0f57861985a296380e4ee7296dac79af
Merge "Fix warning on Special:ListGroupRights for group with invalid group page"