mark unsuccessful vertex fit with out of scope values

reco/KF/CbmKFPrimaryVertexFinder.cxx

@@ -65,8 +65,8 @@ void CbmKFPrimaryVertexFinder::Fit(CbmKFVertexInterface& vtx)
     C[5]        = (t.dz / 2 / 3.5) * (t.dz / 2 / 3.5);
   }
 
+  int fitIterSuccess = 0;  // no. of successful calculations of the vertex position out of requested MaxIter.
   //* Iteratively fit vertex - number of iterations fixed at MaxIter
-
   for (Int_t iteration = 0; iteration < MaxIter; ++iteration) {
 
     //* Store vertex from previous iteration
@@ -194,15 +194,21 @@ void CbmKFPrimaryVertexFinder::Fit(CbmKFVertexInterface& vtx)
       C[3] -= K[2][0] * CHt[0][0] + K[2][1] * CHt[0][1];
       C[4] -= K[2][0] * CHt[1][0] + K[2][1] * CHt[1][1];
       C[5] -= K[2][0] * CHt[2][0] + K[2][1] * CHt[2][1];
-    }  //* itr
-
+    }                                           //* itr
+    if (vtx.GetRefNTracks()) fitIterSuccess++;  // mark success of this iteration for vertex calculation
   }  //* finished iterations
 
   //* Copy state vector to output (covariance matrix was used directly )
-
-  vtx.GetRefX() = r[0];
-  vtx.GetRefY() = r[1];
-  vtx.GetRefZ() = r[2];
+  if (fitIterSuccess) {  // store successful PV calculation. May include also a reference to MaxIter (AB 17.10.2024)
+    vtx.GetRefX() = r[0];
+    vtx.GetRefY() = r[1];
+    vtx.GetRefZ() = r[2];
+  }
+  else {
+    vtx.GetRefX() = -999;
+    vtx.GetRefY() = -999;
+    vtx.GetRefZ() = -999;
+  }
 }
 
 int CbmKFPrimaryVertexFinder::GetUsedTracks(std::vector<uint32_t>& idx) const