Fix compile error on BSD

The cast here should actually be from '0' and not 'NULL' as we are
getting an integer value of 0 for the bbox_idx_count

Fixes https://gitlab.com/kicad/code/kicad/issues/8580

3d-viewer/3d_rendering/legacy/3d_model.cpp

@@ -492,7 +492,7 @@ void MODEL_3D::DrawBbox() const
                     reinterpret_cast<const void*>( offsetof( VERTEX, m_color ) ) );
 
     glDrawElements( GL_LINES, bbox_idx_count, m_bbox_index_buffer_type,
-                    reinterpret_cast<const void*>( NULL ) );
+                    reinterpret_cast<const void*>( 0 ) );
 }