Reset notify parameter in CPWL_ListBox OnDestroy().
Reset notify parameter in CPWL_ListBox OnDestroy().

Currently the OnDestroy() method of CPWL_ListBox will free
the |m_pListNotify| but it does not update the |m_pList| that
it has done so. This causes issues when CPWL_ListBox is
destroyed as the destructor for |m_pList| will attempt to
call into the |m_pListNotify|.

This CL resets the |m_pList| notify parameter before we
clear |m_pListNotify| in OnDestroy so it will not be
accessed during the |m_pList| destructor.

BUG=628995

Review-Url: https://codereview.chromium.org/2160023002

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