ofz#6999 check available data len

Change-Id: I89a7913d35706ee5797beee654ef99be088431b8
Reviewed-on: https://gerrit.libreoffice.org/51499Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit de7d754e)

sw/source/filter/ww8/ww8par4.cxx

@@ -476,8 +476,8 @@ void SwWW8ImplReader::Read_CRevisionMark(RedlineType_t eType,
     const sal_uInt8* pSprmCDttmRMark;
     if( nsRedlineType_t::REDLINE_FORMAT == eType )
     {
-        pSprmCIbstRMark = pData+1;
-        pSprmCDttmRMark = pData+3;
+        pSprmCIbstRMark = nLen >= 3 ? pData+1 : nullptr;
+        pSprmCDttmRMark = nLen >= 7 ? pData+3 : nullptr;
     }
     else
     {