ofz#6173 check index before use

Change-Id: I09d15ab324af0a8b3566f11868eb3266a68c4afe
Reviewed-on: https://gerrit.libreoffice.org/49500Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>
(cherry picked from commit e355d7d6)

sw/source/filter/ww8/ww8toolbar.cxx

@@ -167,7 +167,9 @@ bool SwCTBWrapper::Read( SvStream& rS )
     std::vector< sal_Int16 >::iterator it_end = dropDownMenuIndices.end();
     for ( std::vector< sal_Int16 >::iterator it = dropDownMenuIndices.begin(); it != it_end; ++it )
     {
-        rCustomizations[ *it ].bIsDroppedMenuTB = true;
+        if (*it < 0 || static_cast<size_t>(*it) >= rCustomizations.size())
+            continue;
+        rCustomizations[*it].bIsDroppedMenuTB = true;
     }
     return rS.good();
 }