bugfix: protection for non-existing data source in CbmMCDataArray

Protection for non-existing data source in CbmMCDataArray.

The following code crashes when the "MvdPoint" branch is not present in the data:

CbmMCDataArray *points = CbmMCDataManager::InitBranch("MvdPoint"); // OK, points !=nullptr

points->Size(file, event); // crash