db_block.cpp:  Fix tapos rollover logic
db_block.cpp:  Fix tapos rollover logic

bitshares/bitshares-2,oxarbitrage/bitshares-core,abitmore/bitshares-2,bitshares/bitshares-2,abitmore/bitshares-2,bigoc/openchain,oxarbitrage/bitshares-core,bitsuperlab/cpp-play2,pmconrad/graphene,bitshares/bitshares-2,abitmore/bitshares-2,peertracksinc/muse,pmconrad/graphene,pmconrad/graphene,bitshares/bitshares-2,bitsuperlab/cpp-play2,oxarbitrage/bitshares-core,cryptonomex/graphene,peertracksinc/muse,bitsuperlab/cpp-play2,bigoc/openchain,peertracksinc/muse,cryptonomex/graphene,cryptonomex/graphene,peertracksinc/muse,oxarbitrage/bitshares-core,pmconrad/graphene,bigoc/openchain,bigoc/openchain,abitmore/bitshares-2