In the GetBinError functions, in low statistics bins, do not take into account

non-empty bins with null errors. The effect was to underestimate the error
in case of profiles with many bins with one entry only.


git-svn-id: http://root.cern.ch/svn/root/trunk@7483 27541ba8-7e3a-0410-8455-c3a389f83636