fixed a bug: sieve() must be virtual
fixed a bug: sieve() must be virtual

fredwang00/primesieve,fredwang00/primesieve,ZahidDev/primesieve,kimwalisch/primesieve,anatoliyrazin/primesieve,kimwalisch/primesieve,tempbottle/primesieve,tempbottle/primesieve,tempbottle/primesieve,anatoliyrazin/primesieve,fredwang00/primesieve,kimwalisch/primesieve,anatoliyrazin/primesieve,ZahidDev/primesieve,ZahidDev/primesieve