Fix double delete of packet info objects
Fix for issue ROOT-5055

An object was invalidated going out of scope of the calling function(s)
TProofBench::RunCPU and TProofBench::RunCPUx.
Fix possible double delete at closing

In TProof::Close, fInactiveSlaves was not cleared as the other lists.
Fixes double delete at the end of a TProofBench run.