[Rivet] Yodascale

[Gavin Hesketh]

Hi,
just a small patch for the yodascale script, to avoid crashes when there 
are empty histos (which I found when there are electron and muon channel 
plots in an analysis, but only one channel is filled):

L117 :
         if norm > 0:
             sf = refnorm / norm
         else:
             sf = 0

or something similar.

cheers,
Gavin