Fix crash when trying to draw a schematic polygon with no segments.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/18246

eeschema/sch_painter.cpp

@@ -1491,17 +1491,22 @@ void SCH_PAINTER::draw( const SCH_SHAPE* aShape, int aLayer, bool aDimmed )
                 case SHAPE_T::POLY:
                 {
                     const std::vector<SHAPE*> polySegments = shape->MakeEffectiveShapes( true );
+
+                    if( !polySegments.empty() )
+                    {
                         std::deque<VECTOR2D> pts;
 
                         for( SHAPE* polySegment : polySegments )
                             pts.push_back( static_cast<SHAPE_SEGMENT*>( polySegment )->GetSeg().A );
 
-                    pts.push_back( static_cast<SHAPE_SEGMENT*>( polySegments.back() )->GetSeg().B );
+                        pts.push_back(
+                                static_cast<SHAPE_SEGMENT*>( polySegments.back() )->GetSeg().B );
 
                         for( SHAPE* polySegment : polySegments )
                             delete polySegment;
 
                         m_gal->DrawPolygon( pts );
+                    }
                     break;
                 }