protections for the case CbmEvent::GetNofStsTracks() == -1

protections for the case CbmEvent::GetNofStsTracks() == -1

In the code, it is usually assumed that CbmEvent::GetNofStsTracks() >= 0, while it may be -1.

Actually, CbmEvent::GetNofData(ECbmDataType type) never returns 0.

There is no way to "register" a data type in CbmEvent without adding some data of the type. When there is no data of a type, it returns -1. If there is data, it returns >= 1.

Maybe we should change the convention for CbmEvent::GetNofData(ECbmDataType type) and return 0 when there is no data.