Fix warnings in test/io/test_io.cpp
Fix warnings in test/io/test_io.cpp

> test_io.cpp:799:17: warning: comparison of integers of different signs: 'int' and 'size_type' (aka 'unsigned long') [-Wsign-compare]

shangwuhencc/pcl,DaikiMaekawa/pcl,cascheberg/pcl,nikste/pcl,Tabjones/pcl,RufaelDev/pcc-mp3dg,shivmalhotra/pcl,shyamalschandra/pcl,KevenRing/pcl,DaikiMaekawa/pcl,closerbibi/pcl,chenxingzhe/pcl,soulsheng/pcl,fanxiaochen/mypcltest,damienjadeduff/pcl,drmateo/pcl,simonleonard/pcl,simonleonard/pcl,drmateo/pcl,raydtang/pcl,damienjadeduff/pcl,starius/pcl,mikhail-matrosov/pcl,shyamalschandra/pcl,3dtof/pcl,nikste/pcl,cascheberg/pcl,wgapl/pcl,Tabjones/pcl,the-glu/pcl,KevenRing/vlp,soulsheng/pcl,fskuka/pcl,locnx1984/pcl,mikhail-matrosov/pcl,RufaelDev/pcc-mp3dg,raydtang/pcl,damienjadeduff/pcl,stfuchs/pcl,ResByte/pcl,krips89/pcl_newfeatures,ResByte/pcl,nikste/pcl,wgapl/pcl,LZRS/pcl,shivmalhotra/pcl,locnx1984/pcl,closerbibi/pcl,pkuhto/pcl,lebronzhang/pcl,MMiknis/pcl,shivmalhotra/pcl,stefanbuettner/pcl,ResByte/pcl,wgapl/pcl,kanster/pcl,lydhr/pcl,shyamalschandra/pcl,lebronzhang/pcl,starius/pcl,damienjadeduff/pcl,locnx1984/pcl,stefanbuettner/pcl,sbec/pcl,damienjadeduff/pcl,ipa-rmb/pcl,soulsheng/pcl,KevenRing/pcl,fanxiaochen/mypcltest,fanxiaochen/mypcltest,drmateo/pcl,zhangxaochen/pcl,stefanbuettner/pcl,kanster/pcl,kanster/pcl,drmateo/pcl,zhangxaochen/pcl,mikhail-matrosov/pcl,the-glu/pcl,shyamalschandra/pcl,lebronzhang/pcl,fskuka/pcl,cascheberg/pcl,mschoeler/pcl,ResByte/pcl,closerbibi/pcl,starius/pcl,KevenRing/vlp,krips89/pcl_newfeatures,soulsheng/pcl,sbec/pcl,shangwuhencc/pcl,wgapl/pcl,fanxiaochen/mypcltest,mschoeler/pcl,closerbibi/pcl,simonleonard/pcl,sbec/pcl,sbec/pcl,pkuhto/pcl,fanxiaochen/mypcltest,mikhail-matrosov/pcl,mschoeler/pcl,simonleonard/pcl,MMiknis/pcl,zhangxaochen/pcl,the-glu/pcl,nikste/pcl,stfuchs/pcl,stefanbuettner/pcl,lydhr/pcl,nikste/pcl,wgapl/pcl,lydhr/pcl,Tabjones/pcl,lebronzhang/pcl,locnx1984/pcl,chenxingzhe/pcl,closerbibi/pcl,KevenRing/pcl,Tabjones/pcl,MMiknis/pcl,RufaelDev/pcc-mp3dg,3dtof/pcl,lydhr/pcl,MMiknis/pcl,shangwuhencc/pcl,stfuchs/pcl,pkuhto/pcl,krips89/pcl_newfeatures,krips89/pcl_newfeatures,shangwuhencc/pcl,raydtang/pcl,soulsheng/pcl,ipa-rmb/pcl,zhangxaochen/pcl,fskuka/pcl,cascheberg/pcl,drmateo/pcl,KevenRing/vlp,zhangxaochen/pcl,starius/pcl,3dtof/pcl,LZRS/pcl,fskuka/pcl,KevenRing/vlp,starius/pcl,LZRS/pcl,kanster/pcl,raydtang/pcl,simonleonard/pcl,mschoeler/pcl,Tabjones/pcl,locnx1984/pcl,chenxingzhe/pcl,the-glu/pcl,DaikiMaekawa/pcl,LZRS/pcl,lydhr/pcl,KevenRing/vlp,mschoeler/pcl,shivmalhotra/pcl,pkuhto/pcl,3dtof/pcl,mikhail-matrosov/pcl,RufaelDev/pcc-mp3dg,DaikiMaekawa/pcl,ipa-rmb/pcl,fskuka/pcl,sbec/pcl,stfuchs/pcl,stfuchs/pcl,kanster/pcl,krips89/pcl_newfeatures,stefanbuettner/pcl,ipa-rmb/pcl,shivmalhotra/pcl,shyamalschandra/pcl,ipa-rmb/pcl,raydtang/pcl,pkuhto/pcl,chenxingzhe/pcl,chenxingzhe/pcl,KevenRing/pcl,the-glu/pcl,KevenRing/pcl,shangwuhencc/pcl,MMiknis/pcl,cascheberg/pcl,LZRS/pcl,3dtof/pcl,DaikiMaekawa/pcl,lebronzhang/pcl,ResByte/pcl,RufaelDev/pcc-mp3dg