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/generic/dcpsg.cpp

@@ -1463,6 +1463,8 @@ void wxPostScriptDCImpl::SetLogicalFunction(wxRasterOperationMode WXUNUSED(funct
 void wxPostScriptDCImpl::DoDrawSpline( const wxPointList *points )
 {
     wxCHECK_RET( m_ok, wxT("invalid postscript dc") );
+    wxCHECK_RET(points, "NULL pointer to spline points?");
+    wxCHECK_RET(points->GetCount() >= 2, "incomplete list of spline points?");
 
     SetPen( m_pen );