fork_database.cpp:  Fix overflow
fork_database.cpp:  Fix overflow

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