Null pointer not handled in src/BRep/BRep_Tool.cxx

Hi,

As a FreeCAD contributor I tracked a crash up to src/BRep/BRep_Tool.cxx and discovered that null pointers are not handle in many function.

For example:

Handle(Geom2d_Curve) BRep_Tool::CurveOnSurface(const TopoDS_Edge& E, 
                                               const Handle(Geom_Surface)& S,
                                               const TopLoc_Location& L,
                                               Standard_Real& First,
                                               Standard_Real& Last,
                                               Standard_Boolean* theIsStored)
{
  TopLoc_Location loc = L.Predivided(E.Location());
  Standard_Boolean Eisreversed = (E.Orientation() == TopAbs_REVERSED);
  if (theIsStored)
    *theIsStored = Standard_True;

  // find the representation
  const BRep_TEdge* TE = static_cast<const BRep_TEdge*>(E.TShape().get());
  BRep_ListIteratorOfListOfCurveRepresentation itcr(TE->Curves());

  while (itcr.More()) {
    const Handle(BRep_CurveRepresentation)& cr = itcr.Value();
    if (cr->IsCurveOnSurface(S,loc)) {
      const BRep_GCurve* GC = static_cast<const BRep_GCurve*>(cr.get());
      GC->Range(First,Last);
      if (GC->IsCurveOnClosedSurface() && Eisreversed)
        return GC->PCurve2();
      else
        return GC->PCurve();
    }
    itcr.Next();
  }

  // Curve is not found. Try projection on plane
  if (theIsStored)
    *theIsStored = Standard_False;
  return CurveOnPlane(E, S, L, First, Last);
}

can crash at BRep_ListIteratorOfListOfCurveRepresentation itcr(TE->Curves()); if E shape is not set correctly. Such crash occurs when using BRepFilletAPI_MakeChamfer as shown in this FreeCAD issue.

I still have a PR to propose and wait for a contributor access after I signed the CLA.