fix assert in circular buffer assert caused by race
fix assert in circular buffer assert caused by race

kuszmaul/PerconaFT,percona/PerconaFT,kuszmaul/PerconaFT-tmp,ottok/PerconaFT,BohuTANG/ft-index,percona/PerconaFT,ottok/PerconaFT,kuszmaul/PerconaFT-tmp,kuszmaul/PerconaFT,kuszmaul/PerconaFT,percona/PerconaFT,ottok/PerconaFT,BohuTANG/ft-index,BohuTANG/ft-index,BohuTANG/ft-index,kuszmaul/PerconaFT,percona/PerconaFT,kuszmaul/PerconaFT-tmp,kuszmaul/PerconaFT-tmp,ottok/PerconaFT