Fixed ratsnest related segfaults on BOARD destruction.

pcbnew/class_board.cpp

@@ -110,14 +110,14 @@ BOARD::BOARD() :
 
 BOARD::~BOARD()
 {
-    delete m_ratsnest;
-
     while( m_ZoneDescriptorList.size() )
     {
         ZONE_CONTAINER* area_to_remove = m_ZoneDescriptorList[0];
         Delete( area_to_remove );
     }
 
+    delete m_ratsnest;
+
     m_FullRatsnest.clear();
 
     m_LocalRatsnest.clear();