Fix crash in HcalDigisClient for SLHC configurations

Moved the initialisation of doSLHC_ to before the histograms are booked.

An extra set of histograms are booked in the "booking(...)" method if doSLHC_ is true.  If doSLHC_ is not initialised before these calls then the choice whether to book them or not is made on uninitialised memory.