Trivial Fix for VertexFromTrackProducer

This is a trivial fix for a bug introduced during the "consumes migration" of VertexFromTrackProducer.
Basically the bug was affecting the rate reduction power of isolation @ HLT.

Tracks from edm:Viev<reco::RecoCandidate> > were not properly accessed, preventing from correct generation of vertexes from RecoCandidate tracks.

The net effect in rate reduction for a sample (IsoMu24) path can be summarized as:

Release details / Relative Rate w.r.t. non Iso Path
CMSSW_7_1_0_pre5 (last pre release before the bug was introduced) / relative rate: ~37%
CMSSW_7_1_0_pre9 (before bugfix) / relative rate: ~45%
CMSSW_7_1_0_pre9 (after bugfix) / relative rate: ~37%

@mbluj