Fix warnings in io/openni_device.cpp
Fix warnings in io/openni_device.cpp

> openni_device.cpp:437:29: warning: comparison of integers of different signs: 'pcl::int32_t' (aka 'int') and 'XnUInt32' (aka 'unsigned int') [-Wsign-compare]

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