CheckResid: initialise sigmaYZ of the PV pseudo-cluster

o2::BaseCluster default constructor leaves mSigmaYZ uninitialised, so with
addPVAsCluster the PV cluster entered the refit with a garbage YZ covariance
term, making the residuals non-reproducible between identical runs.

Author: hyungjun-lee

Detectors/GlobalTrackingWorkflow/study/src/CheckResidSpec.cxx

@@ -453,6 +453,7 @@ bool CheckResidSpec::processITSTrack(const o2::its::TrackITS& iTrack, const o2::
     bcPV.setSigmaY2(0.5 * (pv.getSigmaX2() + pv.getSigmaY2()));
     bcPV.setSigmaZ2(pv.getSigmaZ2());
     bcPV.setSensorID(-1);
+    bcPV.setSigmaYZ(0.f); // not initialised by the BaseCluster default constructor
     clArr[0] = &bcPV;
   }
   // collect all track clusters to array, placing them to layer+1 slot