Set internal DCB to NULL on close

The Backend::dcb() method gives the raw pointer to the internal DCB. This
pointer is used by at least readwritesplit to map raw DCB pointers to
backends. To prevent stale pointers from being returned, m_dcb needs to be
set to NULL after it has been closed.

include/maxscale/backend.hh

@@ -158,7 +158,7 @@ public:
     /**
      * @brief Get a pointer to the internal DCB
      *
-     * @return Pointer to internal DCB
+     * @return Pointer to DCB or NULL if not connected
      */
     inline DCB* dcb() const
     {

server/core/backend.cc

@@ -66,6 +66,7 @@ void Backend::close(close_type type)
             }
 
             dcb_close(m_dcb);
+            m_dcb = NULL;
 
             /** decrease server current connection counters */
             atomic_add(&m_backend->connections, -1);