fix wxStringOutputStream::Write() in Unicode build when the output overlaps a boundary between UTF-8 characters (bug 1701426)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@45732 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/sstream.h

@@ -66,6 +66,10 @@ public:
         m_pos = m_str->length() / sizeof(wxChar);
     }
 
+#if wxABI_VERSION >= 20804 && wxUSE_UNICODE
+    virtual ~wxStringOutputStream();
+#endif // wx 2.8.4+
+
     // get the string containing current output
     const wxString& GetString() const { return *m_str; }