Remember to clear pointers when clearing backing data

The snap is a point to the anchors that we compute.  This needs to be
removed when we clear the vector

Fixes https://gitlab.com/kicad/code/kicad/issues/13037

include/tool/grid_helper.h

@@ -112,6 +112,7 @@ protected:
     void clearAnchors()
     {
         m_anchors.clear();
+        m_snapItem = nullptr;
     }
 
     /**