Pcbnew: fix assertion when deleting BOARD_ITEM not linked to list. (fixes lp:1252696)

include/class_board_item.h

@@ -172,11 +172,13 @@ public:
 
     /**
      * Function DeleteStructure
-     * deletes this object after UnLink()ing it from its owner.
+     * deletes this object after UnLink()ing it from its owner if it has one.
      */
     void DeleteStructure()
     {
+        if( GetList() != NULL )
             UnLink();
+
         delete this;
     }