Always delete the array when the array size goes down to zero.

This fixes a problem seen by CMS when using TH1::Copy on histograms read
from a TTree.

This amend commit d31f38f98c66af492c20c3bf060df13d64c465f0 (circa 2005)
Merge pull request #47 from Dr15Jones/philippeFixEmptyArrayIO_v5-34-22

Always delete the array when the array size goes down to zero.