[Sketcher] Do regular drag for small B-splines

Here, "small" means that the number of poles of the spline is so low that moving
any piece of the curve without changing shape would require moving all the
poles. In that case the rest of the algorithm in `initBSplinePieceMove()` only
complicates the matter.

src/Mod/Sketcher/App/Sketch.cpp

@@ -4040,11 +4040,16 @@ int Sketch::initBSplinePieceMove(int geoId, PointPos pos, const Base::Vector3d&
         return -1;
     }
 
+    GCS::BSpline &bsp = BSplines[Geoms[geoId].index];
+
+    // If spline has too few poles, just move all
+    if (bsp.poles.size() <= (bsp.degree + 1))
+        return initMove(geoId, pos, fine);
+
     // Find the closest knot
     auto partBsp = static_cast<GeomBSplineCurve*>(Geoms[geoId].geo);
     double uNear;
     partBsp->closestParameter(firstPoint, uNear);
-    GCS::BSpline &bsp = BSplines[Geoms[geoId].index];
     auto& knots = bsp.knots;
     auto upperknot = std::upper_bound(
         knots.begin(), knots.end(), uNear,