Fix copy constructor implementation of BinData
Fix copy constructor implementation of BinData

This fixes problem seen in
https://root-forum.cern.ch/t/globalchi2-function-for-many-histograms-in-root-fitfcn/42016/10
and it can be reproduced with this simple code:
```
{
 std::vector<ROOT::Fit::BinData> data;
  ROOT::Fit::DataOptions opt;
  ROOT::Fit::DataRange> range;
  for(int k=0;k<3;k++){ data.push_back(ROOT::Fit::BinData(opt,range)); }
}
```

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