libreoffice/ww6fuzzer: Bad-cast to SwIndexReg from invalid vptr in SwIndex::Remove

Detailed report: https://oss-fuzz.com/testcase?key=6290151587184640 Project: libreoffice Fuzzer: libFuzzerlibreofficeww6fuzzer Fuzz target binary: ww6fuzzer Job Type: libfuzzerubsanlibreoffice Platform Id: linux Crash Type: Bad-cast Crash Address: 0x000012e6bed8 Crash State: Bad-cast to SwIndexRe...