Make some examples free their memory when finished

src/mobility/examples/main-grid-topology.cc

@@ -50,5 +50,6 @@ int main (int argc, char *argv[])
       std::cout << "x=" << pos.x << ", y=" << pos.y << ", z=" << pos.z << std::endl;
     }
 
+  Simulator::Destroy ();
   return 0;
 }

src/mobility/examples/main-random-topology.cc

@@ -37,5 +37,6 @@ int main (int argc, char *argv[])
 
   Simulator::Run ();
   
+  Simulator::Destroy ();
   return 0;
 }

src/mobility/examples/main-random-walk.cc

@@ -46,5 +46,6 @@ int main (int argc, char *argv[])
 
   Simulator::Run ();
   
+  Simulator::Destroy ();
   return 0;
 }