Don't allow negative line widths for grahpic shapes.

Fixes https://gitlab.com/kicad/code/kicad/issues/5599

pcbnew/dialogs/dialog_graphic_item_properties.cpp

@@ -256,6 +256,9 @@ bool DIALOG_GRAPHIC_ITEM_PROPERTIES::TransferDataFromWindow()
     if( !DIALOG_GRAPHIC_ITEM_PROPERTIES_BASE::TransferDataFromWindow() )
         return false;
 
+    if( !m_thickness.Validate( 0, Millimeter2iu( 1000.0 ) ) )
+        return false;
+
     LAYER_NUM layer = m_LayerSelectionCtrl->GetLayerSelection();
 
     BOARD_COMMIT commit( m_parent );