Fix event loop in RecoTzero. Refs #2689.

CbmRecoTzero took the TZD digis directly from the timeslice array. In case of time-based simulation, when there is more than one event in the timeslice, this lead to an incorrect determination of the number of TZD digis in the event. Consequently, the event tzero was set to -999999 (error code).

This is fixed in this MR, by properly addressing the TZD digis through the CbmEvent objects.