EvtDecayProb minor fixes

Summary: Initialise data members & remove empty destructor in EvtDecayProb.hh

Test Plan: The changes provide identical distributions for decay prob models such as EvtPi0Dalitz.

Reviewers: kreps, tlatham

Reviewed By: tlatham

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D82