Use TObjArray::GetEntriesFast to avoid wasting time.

TObjArray::GetEntries() counts the number of non-zero slots
while TObjArray::GetEntriesFast() return (in most case) the value of a high water mark (the slot over which all remaining slots
are empty).

In one use case (doing read a TTree and do many TTree cloning (via TBufferMerger), TObjArray::GetEntries was taking 8% of the
running time (compression was disabled)

Note: TObjArray::IsEmpty use GetEntriesFast rather than GetEntries.