Fixed uninitialized variable in PFTau3ProngSummary

PFTau3ProngSummary::Tau_Charge was returning an undefined value
since it was using a uninitialized stack variable as its charge
summing variable. The problem was found by the static analyzer.
Merge pull request #3436 from Dr15Jones/fixUninitializedVariableInPFTau3ProngSummary

DataFormats/TauReco -- Fixed uninitialized variable in PFTau3ProngSummary