Move adjustment of revision delete target in SpecialRevisiondelete to fix fatal

targetObj needs to be set correctly before accessing the RevisionList,
and trying to get the 'current'.  If not set correctly, then $title
given to RevisionList (and it's subclasses) is Special:Revisiondelete
and rev_page is 0. So nothing is found for 'current' and a fatal
error happens.

Follow-up to Id1baacb

Bug: 68566
Change-Id: Ic078af5417798db1b2900057dbf7514112e94b46