Add missing write lock guard in TObjArray::GetAbsLast.
Add missing write lock guard in TObjArray::GetAbsLast.

This fixes ROOT-10057

karies/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root