message: memory manger returns null pointer if you give it one
diff: @@ -251,7 +251,11 @@ void *mm::Ptr(void *ptr)
  if (MmDeviceIniFilter()) { return ptr; }
  if (Known(ptr)) { return PtrKnown(maps, ptr); }
  if (Alias(ptr)) { return PtrAlias(maps, ptr); }
  - if (config::UsingDevice())
  + if (config::UsingDevice() && ptr==NULL)
  + {
  + return NULL;
  + }
  + else
  {
  mfem_error("Trying to use unknown pointer on the DEVICE!");
  }
  