CA: a protection for the null ptr

reco/L1/catools/CaToolsMaterialHelper.cxx

@@ -206,7 +206,11 @@ L1Material MaterialHelper::GenerateMaterialMap(double zRef, double zMin, double
             bool doContinue = 1;
             for (int iStep = 0; doContinue; iStep++) {
 
-              if (!node) { LOG(fatal) << "ca::tools::MaterialHelper: TGeoNavigator can not find the geo node"; }
+              if (!node) {
+                // may happen when tracing outside of the CBM volume -> produce a warning
+                LOG(warning) << "ca::tools::MaterialHelper: TGeoNavigator can not find the geo node";
+                break;
+              }
 
               TGeoMedium* medium = node->GetMedium();
               if (!medium) { LOG(fatal) << "ca::tools::MaterialHelper: TGeoNavigator can not find the geo medium"; }