fixed getters to be able to read from miniAOD

Bug fix for the pat::Tau accessor functions that use pfSpecific(). This function (rightly) returns cms::error, when ones tried to read from miniAOD because pfSpecific is not filled by patTauSlimmer. This bugfix introduces check for the existence of pfSpecific before accessing it.

Tau POG considers this bugfix critical for CSA14 - without it the analysis of taus in miniAOD will not be possible.

No change in physics output is expected.

@monicava, @veelken