proper member initialization

allocating memory of zero size will produce useless pointer
set to NULL instead

src/lagrangian/cfdemParticle/subModels/forceModel/forceModel/forceModel.C

@@ -88,7 +88,7 @@ forceModel::forceModel
     probeIt_(sm.probeM().active()),
     requiresEx_(false),
     forceSubModels_(0),
-    forceSubModel_(new autoPtr<forceSubModel>[nrForceSubModels()])
+    forceSubModel_(NULL)
 {}