POLYGON_BOOLEAN_ROUTINE: init some variables

These should be inited later on, but it's clearer to
make it explicit.

pcbnew/tools/item_modification_routine.cpp

@@ -528,6 +528,10 @@ void POLYGON_BOOLEAN_ROUTINE::Finalize()
     // new PCB_SHAPEs for each outline
     for( int i = 0; i < m_workingPolygons.OutlineCount(); ++i )
     {
+        // If we handled any polygons to get any outline,
+        // there must be a layer set by now.
+        wxASSERT( m_layer >= 0 );
+
         std::unique_ptr<PCB_SHAPE> new_poly_shape =
                 std::make_unique<PCB_SHAPE>( GetBoard(), SHAPE_T::POLY );
 

pcbnew/tools/item_modification_routine.h

@@ -358,8 +358,8 @@ private:
     SHAPE_POLY_SET m_workingPolygons;
 
     bool         m_firstPolygon = true;
-    int          m_width;
-    PCB_LAYER_ID m_layer;
+    int          m_width = 0;
+    PCB_LAYER_ID m_layer = PCB_LAYER_ID::UNDEFINED_LAYER;
 };
 
 class POLYGON_MERGE_ROUTINE : public POLYGON_BOOLEAN_ROUTINE