Check that number of points passed to wxDC::DrawSpline() is at least 2

DoDrawSpline() implementations for all ports work for the number of points
>= 2 (for 2 points there is drawn a straight line) so we need to add checks
whether this requirement is met.

See #19172.

src/gtk/print.cpp

@@ -1716,6 +1716,9 @@ void wxGtkPrinterDCImpl::DoDrawEllipse(wxCoord x, wxCoord y, wxCoord width, wxCo
 #if wxUSE_SPLINES
 void wxGtkPrinterDCImpl::DoDrawSpline(const wxPointList *points)
 {
+    wxCHECK_RET(points, "NULL pointer to spline points?");
+    wxCHECK_RET(points->GetCount() >= 2, "incomplete list of spline points?");
+
     SetPen (m_pen);
 
     double c, d, x1, y1, x3, y3;