qlockfile_unix - code cleanup

Coverity's CID 157687: QCache::insert, indeed, can delete (immediately)
the object we're trying to insert. While this never happens actually in
qlockfile_unix since we have max cost 10 and insert with cost 1, the
code does not look good and Coverity is not happy.

Change-Id: I16a428017bf86e151afe5256906e4cab1ef4044a
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>