x/mobile/app: zero buffer at end of read
x/mobile/app: zero buffer at end of read

LGTM=hyangah
R=hyangah
CC=an.olive.tree, golang-codereviews
https://golang.org/cl/176130045

ppamorim/mobile,samanalysis/mobile,MaxCy330/mobile,xing/gomobile,0x90sled/mobile,golang/mobile,0x90sled/mobile,b54898533/mobile,steeve/mobile,steeve/mobile,goldcaddy77/mobile,lonphy23t/mobile,0x90sled/mobile,ichu501/mobile,grategames/mobile,goldcaddy77/mobile,DanielWorld/mobile,cainiaocome/mobile,tempbottle/mobile,DanielWorld/mobile,DanielWorld/mobile,MaxCy330/mobile,cainiaocome/mobile,xing/gomobile,zach-klippenstein/mobile,golang/mobile,ichu501/mobile,lonphy23t/mobile,grategames/mobile,lonphy23t/mobile,b54898533/mobile,qskycolor/mobile,b54898533/mobile,golang/mobile,ppamorim/mobile,zach-klippenstein/mobile,grategames/mobile,ichu501/mobile,qskycolor/mobile,tempbottle/mobile,swisspol/mobile,ppamorim/mobile,samanalysis/mobile,goldcaddy77/mobile,steeve/mobile,steeve/mobile,ichu501/mobile,lonphy23t/mobile,samanalysis/mobile,freeman0211/mobile,b54898533/mobile,cainiaocome/mobile,cainiaocome/mobile,tempbottle/mobile,grategames/mobile,xing/gomobile,qskycolor/mobile,samanalysis/mobile,freeman0211/mobile,MaxCy330/mobile,freeman0211/mobile,freeman0211/mobile,goldcaddy77/mobile,goldcaddy77/mobile,swisspol/mobile,0x90sled/mobile,zach-klippenstein/mobile,DanielWorld/mobile,tempbottle/mobile,swisspol/mobile,xing/gomobile,qskycolor/mobile,swisspol/mobile,tempbottle/mobile,ppamorim/mobile,MaxCy330/mobile,swisspol/mobile,samanalysis/mobile,0x90sled/mobile,MaxCy330/mobile,xing/gomobile,golang/mobile,zach-klippenstein/mobile,lonphy23t/mobile,cainiaocome/mobile,ppamorim/mobile,b54898533/mobile,ichu501/mobile