The previous implementation os vtkSEPReader had a use-after-free.

The function `vtkSEPReader::ReplaceFileName(..)` started with:

```c++
  delete[] this->FileName;
```

And that means that the string pointed by `FileName` was
destroyed. Even if the value of `FileName` was stored in the local
variable `fileName`, in `vtkSEPReader::RequestData`, the string
pointed by it was destroyed!

The patch of this commit is a lot simpler: both the string pointed by
`FileName` and the one that will replace it already exist in memory,
and will not be modified during the run of `RequestData`. So we can
just swap the pointers, without any need to memory
allocation/deallocation to copy the strings.