DXF reader stalls

The DXF reader stalls when the knot intervals are negative to 0 for spline interpolation. 

The reason is a incorrect negative step distance which leads to an infinite loop.

This is the wrong formula:

```
  //  dbDXFReader.cc: 916++
  double dt = 0.5 * (tn + t0);
```

The formula should be "tn - t0".
