pkg/tar: tar_test.go - fixing possible nil pointer dereference
pkg/tar: tar_test.go - fixing possible nil pointer dereference

Playing a little with tar.go caused a panic in tar_test.go. This fixes it.
ChengTiesheng/rkt,joshix/rkt,sinfomicien/rkt,cab105/rkt,wparad/rkt,fdserr/rkt,chrisseto/rkt,smarvin/rkt,rhvgoyal/rkt,luebken/rkt,maquanyi/rkt,squeed/rkt,tmrts/rkt,alexhersh/rkt,tv42/rkt,sstabellini/rkt,ramitsurana05/rkt,mattthias/pkg-rkt,dgonyeo/rkt,joshix/rkt,markllama/rkt,lucasallan/rkt,monstermunchkin/rkt,intfrr/rkt,sgotti/rkt,sstabellini/rkt,lee212/rkt,bsmr-go/rkt,bsmr-go/rkt,chrisseto/rkt,sgotti/rkt,fdserr/rkt,shtouff/rkt,clearlinux/rkt,nhlfr/rkt,iaguis/rkt,endocode/rkt,robszumski/rocket,intfrr/rkt,marineam/rkt,s-urbaniak/rkt,achanda/rkt,ChengTiesheng/rkt,xnox/rkt,alexhersh/rkt,iaguis/rkt,MattDees/rkt,joshix/rkt,markllama/rkt,yifan-gu/rocket,il9ue/rkt,coreos/rkt,endocode/rkt,mike1729/rkt,Wilybald/rkt,MattDees/rkt,Wilybald/rkt,mike1729/rkt,Wilybald/rkt,rot26/rkt,liubq919/rkt,marineam/rkt,chrisseto/rkt,squaremo/rkt,mattthias/pkg-rkt,sgotti/rkt,s-urbaniak/rkt,fdserr/rkt,alban/rkt,liugenping/rkt,yifan-gu/rkt,marineam/rkt,bsmr-go/rkt,eyakubovich/rkt,rkt/rkt,Shashwatsh/rkt,tmrts/rkt,tmrts/rkt,cab105/rkt,jzelinskie/rkt,kbrwn/rkt,kbrwn/rkt,coreos/rkt,Shashwatsh/rkt,matthaias/rkt,tixxdz/rkt,luebken/rkt,sstabellini/rkt,michelangelo13/rkt,ramitsurana05/rkt,tixxdz/rkt,yifan-gu/rkt,squeed/rkt,yifan-gu/rkt,tv42/rkt,Wilybald/rkt,rkt/rkt,tv42/rkt,mike1729/rkt,ramitsurana/rkt,rot26/rkt,lsm5/rkt,shobhit85/rkt,alban/rkt,lsm5/rkt,yifan-gu/rocket,cab105/rkt,rot26/rkt,shtouff/rkt,klizhentas/rkt,intelsdi-x/rkt,monstermunchkin/rkt,jakon89/rkt,liubq919/rkt,nhlfr/rkt,iankronquist/rkt,ramitsurana05/rkt,jonboulle/rkt,adfernandes/rkt,markllama/rkt,shobhit85/rkt,adfernandes/rkt,lee212/rkt,maquanyi/rkt,robszumski/rocket,jonboulle/rkt,rkt/rkt,mjg59/rkt,intelsdi-x/rkt,kbrwn/rkt,blixtra/rkt,nhlfr/rkt,crawford/rkt,jjlakis/rkt,iankronquist/rkt,alban/rkt,eyakubovich/rkt,squaremo/rkt,monstermunchkin/rkt,jasonchaffee/rkt,smarvin/rkt,shtouff/rkt,euank/rkt,klizhentas/rkt,rkt/rkt,euank/rkt,liugenping/rkt,squaremo/rkt,krnowak/rkt,iankronquist/rkt,michelangelo13/rkt,sinfomicien/rkt,dgonyeo/rkt,jasonchaffee/rkt,jakon89/rkt,lucasallan/rkt,linki/rkt,dalanlan/rkt,lee212/rkt,ramitsurana/rkt,linki/rkt,jzelinskie/rkt,smarvin/rkt,achanda/rkt,kinvolk/rkt,ybubnov/rkt,jasonchaffee/rkt,xnox/rkt,kinvolk/rkt,robszumski/rocket,Yhgenomics/rkt,sinfomicien/rkt,MabinGo/rkt,luebken/rkt,ybubnov/rkt,ChengTiesheng/rkt,MabinGo/rkt,jzelinskie/rkt,mattthias/pkg-rkt,il9ue/rkt,xnox/rkt,squeed/rkt,lucasallan/rkt,klizhentas/rkt,euank/rkt,dalanlan/rkt,MattDees/rkt,liubq919/rkt,MabinGo/rkt,jjlakis/rkt,endocode/rkt,lucasallan/rkt,linki/rkt,krnowak/rkt,Yhgenomics/rkt,rhvgoyal/rkt,dgonyeo/rkt,krnowak/rkt,mjg59/rkt,tixxdz/rkt,iaguis/rkt,wparad/rkt,adfernandes/rkt,kinvolk/rkt,joshix/rkt,alexhersh/rkt,crawford/rkt,clearlinux/rkt,jjlakis/rkt,liugenping/rkt,akshaykarle/rkt,eyakubovich/rkt,maquanyi/rkt,ybubnov/rkt,jonboulle/rkt,lsm5/rkt,achanda/rkt,s-urbaniak/rkt,yifan-gu/rocket,intelsdi-x/rkt,il9ue/rkt,intfrr/rkt,clearlinux/rkt,Yhgenomics/rkt,ramitsurana/rkt,crawford/rkt,kinvolk/rkt,Shashwatsh/rkt,blixtra/rkt,matthaias/rkt,blixtra/rkt,akshaykarle/rkt,dalanlan/rkt,michelangelo13/rkt,shobhit85/rkt,matthaias/rkt,rhvgoyal/rkt,akshaykarle/rkt,jakon89/rkt,mjg59/rkt,wparad/rkt,coreos/rkt