Fix warnings in io/openni_grabber.cpp
Fix warnings in io/openni_grabber.cpp

> openni_grabber.cpp:598:21: warning: comparison of integers of different signs: 'int' and 'const unsigned int' [-Wsign-compare]
> openni_grabber.cpp:600:32: warning: comparison of integers of different signs: 'int' and 'const unsigned int' [-Wsign-compare]
> openni_grabber.cpp:703:21: warning: comparison of integers of different signs: 'int' and 'const unsigned int' [-Wsign-compare]
> openni_grabber.cpp:705:32: warning: comparison of integers of different signs: 'int' and 'const unsigned int' [-Wsign-compare]
> openni_grabber.cpp:814:21: warning: comparison of integers of different signs: 'int' and 'const unsigned int' [-Wsign-compare]
> openni_grabber.cpp:816:21: warning: comparison of integers of different signs: 'int' and 'const unsigned int' [-Wsign-compare]

mikhail-matrosov/pcl,Tabjones/pcl,nikste/pcl,zhangxaochen/pcl,fskuka/pcl,stefanbuettner/pcl,ResByte/pcl,stfuchs/pcl,jakobwilm/pcl,nikste/pcl,3dtof/pcl,soulsheng/pcl,zhangxaochen/pcl,ipa-rmb/pcl,wgapl/pcl,chenxingzhe/pcl,zhangxaochen/pcl,soulsheng/pcl,zavataafnan/pcl-truck,wgapl/pcl,fanxiaochen/mypcltest,chatchavan/pcl,LZRS/pcl,zavataafnan/pcl-truck,mschoeler/pcl,pkuhto/pcl,DaikiMaekawa/pcl,srbhprajapati/pcl,jakobwilm/pcl,damienjadeduff/pcl,closerbibi/pcl,simonleonard/pcl,lydhr/pcl,fanxiaochen/mypcltest,stefanbuettner/pcl,RufaelDev/pcc-mp3dg,shangwuhencc/pcl,raydtang/pcl,fanxiaochen/mypcltest,MMiknis/pcl,jeppewalther/kinfu_segmentation,KevenRing/vlp,cascheberg/pcl,drmateo/pcl,raydtang/pcl,srbhprajapati/pcl,v4hn/pcl,mikhail-matrosov/pcl,lydhr/pcl,srbhprajapati/pcl,mschoeler/pcl,ResByte/pcl,stfuchs/pcl,stfuchs/pcl,zhangxaochen/pcl,the-glu/pcl,mikhail-matrosov/pcl,damienjadeduff/pcl,cascheberg/pcl,nikste/pcl,msalvato/pcl_kinfu_highres,stfuchs/pcl,shivmalhotra/pcl,lydhr/pcl,fanxiaochen/mypcltest,shivmalhotra/pcl,MMiknis/pcl,mschoeler/pcl,mschoeler/pcl,KevenRing/pcl,MMiknis/pcl,shyamalschandra/pcl,lebronzhang/pcl,soulsheng/pcl,jeppewalther/kinfu_segmentation,KevenRing/pcl,fskuka/pcl,shyamalschandra/pcl,v4hn/pcl,fskuka/pcl,stfuchs/pcl,damienjadeduff/pcl,locnx1984/pcl,ResByte/pcl,starius/pcl,soulsheng/pcl,stefanbuettner/pcl,sbec/pcl,lydhr/pcl,raydtang/pcl,LZRS/pcl,jeppewalther/kinfu_segmentation,jeppewalther/kinfu_segmentation,shyamalschandra/pcl,DaikiMaekawa/pcl,jeppewalther/kinfu_segmentation,damienjadeduff/pcl,zavataafnan/pcl-truck,krips89/pcl_newfeatures,LZRS/pcl,the-glu/pcl,closerbibi/pcl,KevenRing/pcl,starius/pcl,ResByte/pcl,nikste/pcl,chatchavan/pcl,3dtof/pcl,locnx1984/pcl,closerbibi/pcl,kanster/pcl,the-glu/pcl,shangwuhencc/pcl,simonleonard/pcl,lebronzhang/pcl,RufaelDev/pcc-mp3dg,wgapl/pcl,lebronzhang/pcl,LZRS/pcl,drmateo/pcl,wgapl/pcl,simonleonard/pcl,LZRS/pcl,RufaelDev/pcc-mp3dg,Tabjones/pcl,wgapl/pcl,shangwuhencc/pcl,lebronzhang/pcl,ipa-rmb/pcl,pkuhto/pcl,drmateo/pcl,KevenRing/vlp,srbhprajapati/pcl,chatchavan/pcl,chatchavan/pcl,RufaelDev/pcc-mp3dg,v4hn/pcl,nikste/pcl,locnx1984/pcl,srbhprajapati/pcl,3dtof/pcl,lydhr/pcl,chenxingzhe/pcl,zhangxaochen/pcl,DaikiMaekawa/pcl,krips89/pcl_newfeatures,raydtang/pcl,msalvato/pcl_kinfu_highres,v4hn/pcl,pkuhto/pcl,kanster/pcl,shivmalhotra/pcl,zavataafnan/pcl-truck,msalvato/pcl_kinfu_highres,drmateo/pcl,ipa-rmb/pcl,drmateo/pcl,v4hn/pcl,starius/pcl,krips89/pcl_newfeatures,chenxingzhe/pcl,shangwuhencc/pcl,fskuka/pcl,KevenRing/vlp,3dtof/pcl,shivmalhotra/pcl,sbec/pcl,DaikiMaekawa/pcl,jakobwilm/pcl,jakobwilm/pcl,lebronzhang/pcl,soulsheng/pcl,chenxingzhe/pcl,KevenRing/vlp,locnx1984/pcl,mikhail-matrosov/pcl,cascheberg/pcl,stefanbuettner/pcl,KevenRing/pcl,mschoeler/pcl,ResByte/pcl,msalvato/pcl_kinfu_highres,cascheberg/pcl,KevenRing/vlp,sbec/pcl,chenxingzhe/pcl,Tabjones/pcl,DaikiMaekawa/pcl,MMiknis/pcl,starius/pcl,krips89/pcl_newfeatures,fskuka/pcl,shyamalschandra/pcl,the-glu/pcl,krips89/pcl_newfeatures,jakobwilm/pcl,shivmalhotra/pcl,starius/pcl,pkuhto/pcl,sbec/pcl,pkuhto/pcl,mikhail-matrosov/pcl,cascheberg/pcl,MMiknis/pcl,kanster/pcl,damienjadeduff/pcl,kanster/pcl,locnx1984/pcl,closerbibi/pcl,3dtof/pcl,simonleonard/pcl,zavataafnan/pcl-truck,RufaelDev/pcc-mp3dg,shangwuhencc/pcl,kanster/pcl,the-glu/pcl,shyamalschandra/pcl,Tabjones/pcl,ipa-rmb/pcl,raydtang/pcl,Tabjones/pcl,fanxiaochen/mypcltest,ipa-rmb/pcl,msalvato/pcl_kinfu_highres,sbec/pcl,stefanbuettner/pcl,simonleonard/pcl,KevenRing/pcl,closerbibi/pcl