fix underflow
fix underflow

Wish c++ could specify saturating unsigned underflow.
Howaner/MCServer,thetaeo/cuberite,zackp30/cuberite,birkett/cuberite,Haxi52/cuberite,nevercast/cuberite,mjssw/cuberite,nichwall/cuberite,johnsoch/cuberite,mmdk95/cuberite,mmdk95/cuberite,Haxi52/cuberite,mjssw/cuberite,mc-server/MCServer,nounoursheureux/MCServer,bendl/cuberite,linnemannr/MCServer,QUSpilPrgm/cuberite,marvinkopf/cuberite,Schwertspize/cuberite,Howaner/MCServer,ionux/MCServer,guijun/MCServer,Fighter19/cuberite,guijun/MCServer,Haxi52/cuberite,kevinr/cuberite,tonibm19/cuberite,ionux/MCServer,Tri125/MCServer,Howaner/MCServer,Tri125/MCServer,HelenaKitty/EbooMC,Haxi52/cuberite,ionux/MCServer,bendl/cuberite,nicodinh/cuberite,jammet/MCServer,Frownigami1/cuberite,mmdk95/cuberite,mmdk95/cuberite,HelenaKitty/EbooMC,QUSpilPrgm/cuberite,ionux/MCServer,birkett/MCServer,Tri125/MCServer,nichwall/cuberite,tonibm19/cuberite,birkett/MCServer,birkett/cuberite,SamOatesPlugins/cuberite,nounoursheureux/MCServer,kevinr/cuberite,nicodinh/cuberite,kevinr/cuberite,QUSpilPrgm/cuberite,Haxi52/cuberite,Schwertspize/cuberite,Schwertspize/cuberite,HelenaKitty/EbooMC,electromatter/cuberite,mc-server/MCServer,linnemannr/MCServer,Fighter19/cuberite,marvinkopf/cuberite,guijun/MCServer,QUSpilPrgm/cuberite,ionux/MCServer,nevercast/cuberite,zackp30/cuberite,Altenius/cuberite,bendl/cuberite,mmdk95/cuberite,Frownigami1/cuberite,thetaeo/cuberite,Altenius/cuberite,johnsoch/cuberite,linnemannr/MCServer,Tri125/MCServer,johnsoch/cuberite,zackp30/cuberite,Howaner/MCServer,birkett/cuberite,nevercast/cuberite,SamOatesPlugins/cuberite,thetaeo/cuberite,nichwall/cuberite,nounoursheureux/MCServer,mc-server/MCServer,linnemannr/MCServer,nicodinh/cuberite,jammet/MCServer,tonibm19/cuberite,johnsoch/cuberite,birkett/cuberite,electromatter/cuberite,Howaner/MCServer,Haxi52/cuberite,Frownigami1/cuberite,zackp30/cuberite,Frownigami1/cuberite,mjssw/cuberite,thetaeo/cuberite,mjssw/cuberite,mc-server/MCServer,nevercast/cuberite,nicodinh/cuberite,birkett/MCServer,zackp30/cuberite,kevinr/cuberite,Altenius/cuberite,nichwall/cuberite,bendl/cuberite,birkett/cuberite,zackp30/cuberite,kevinr/cuberite,tonibm19/cuberite,nounoursheureux/MCServer,thetaeo/cuberite,electromatter/cuberite,Altenius/cuberite,tonibm19/cuberite,bendl/cuberite,mjssw/cuberite,thetaeo/cuberite,Fighter19/cuberite,electromatter/cuberite,jammet/MCServer,electromatter/cuberite,nevercast/cuberite,Tri125/MCServer,mjssw/cuberite,jammet/MCServer,Fighter19/cuberite,Frownigami1/cuberite,SamOatesPlugins/cuberite,birkett/cuberite,Howaner/MCServer,nevercast/cuberite,marvinkopf/cuberite,SamOatesPlugins/cuberite,mmdk95/cuberite,nicodinh/cuberite,ionux/MCServer,marvinkopf/cuberite,guijun/MCServer,birkett/MCServer,marvinkopf/cuberite,Fighter19/cuberite,tonibm19/cuberite,HelenaKitty/EbooMC,kevinr/cuberite,QUSpilPrgm/cuberite,mc-server/MCServer,jammet/MCServer,nounoursheureux/MCServer,jammet/MCServer,birkett/MCServer,guijun/MCServer,johnsoch/cuberite,linnemannr/MCServer,HelenaKitty/EbooMC,birkett/MCServer,nicodinh/cuberite,nichwall/cuberite,nounoursheureux/MCServer,Schwertspize/cuberite,mc-server/MCServer,electromatter/cuberite,QUSpilPrgm/cuberite,SamOatesPlugins/cuberite,linnemannr/MCServer,marvinkopf/cuberite,Fighter19/cuberite,nichwall/cuberite,Schwertspize/cuberite,Altenius/cuberite,Tri125/MCServer,guijun/MCServer