Modeling Algorithms - use the adaptor's NbPoles() in BRepGProp_EdgeTool::IntegrationOrder

[gsdali]

Fixes #1381.

Problem

BRepGProp::LinearProperties() (and anything built on BRepGProp_Cinert) SIGSEGVs computing the
integration order for an edge whose only curve geometry is a Bezier- or BSpline-type
curve-on-surface (pcurve) — an edge with no 3D curve, the common case for a degenerate edge
BRepBuilderAPI_Sewing produces reconciling near-coincident vertices between two faces that don't
share an edge outright.

Root cause

IntegrationOrder branches on BAC.GetType(), which correctly reports the curve-on-surface
pcurve's type via GeomAdaptor_TransformedCurve::GetType()'s override:

return myConSurf.IsNull() ? myCurve.GetType() : myConSurf->GetType();

but then read the pole count via a completely different, non-virtual path — BAC.Curve().Curve(),
down-cast to Geom_BezierCurve/Geom_BSplineCurve. BAC.Curve() returns the base
GeomAdaptor_Curve sub-object, which holds the 3D-curve representation only: it is never Load()ed
when the edge has no 3D curve, so the handle is null, the down-cast returns null, and ->NbPoles()
dereferences it.

Fix

GeomAdaptor_TransformedCurve already has a correctly-dispatching NbPoles() override right next
to GetType() in the same header:

int NbPoles() const override
{
  return myConSurf.IsNull() ? myCurve.NbPoles() : myConSurf->NbPoles();
}

Calling BAC.NbPoles() instead of manually re-deriving the pole count from BAC.Curve() fixes the
crash and matches the accessor GetType() one line above it already uses — no cast, no null check
needed, and no behaviour change for an edge that does have a 3D curve (myConSurf.IsNull() there,
so NbPoles() resolves to the same myCurve.NbPoles() the removed code reached by hand).

Reproducer / test

Handle(Geom_Plane) plane = new Geom_Plane(gp_Pnt(0, 0, 0), gp_Dir(0, 0, 1));

TColgp_Array1OfPnt2d poles(1, 4);
poles.SetValue(1, gp_Pnt2d(0.0, 0.0));
poles.SetValue(2, gp_Pnt2d(0.1, 0.05));
poles.SetValue(3, gp_Pnt2d(0.2, -0.05));
poles.SetValue(4, gp_Pnt2d(0.3, 0.0));

TColStd_Array1OfReal knots(1, 2);
knots.SetValue(1, 0.0);
knots.SetValue(2, 1.0);

TColStd_Array1OfInteger mults(1, 2);
mults.SetValue(1, 4);
mults.SetValue(2, 4);

Handle(Geom2d_BSplineCurve) pcurve = new Geom2d_BSplineCurve(poles, knots, mults, 3);

BRep_Builder builder;
TopoDS_Edge edge;
builder.MakeEdge(edge);
builder.UpdateEdge(edge, pcurve, plane, TopLoc_Location(), Precision::Confusion());
builder.Range(edge, plane, TopLoc_Location(), pcurve->FirstParameter(), pcurve->LastParameter());
builder.Degenerated(edge, true);

GProp_GProps props;
BRepGProp::LinearProperties(edge, props);  // crashes on master

Added as BRepGPropTest.LinearProperties_DegenerateEdgeWithBSplinePCurveNoCrash in the existing
BRepGProp_Test.cxx.