Silence aliasing warning.
Fix for the PROOF sub-merger mode

When creating mergers, the code iterates over all slaves to figure out how
many workers should be assigned to each merger. This is usually fine,
unless not all are active. In that situation it creates mergers that wait
forever (or almost forever) for workers which will never connect, which
obviously screws up the job. The fix changes the merger creation to iterate
over fActiveSlaves instead of fSlaves.

[From B.Butler, M.Swiatlowski]