coverity: don't risk exec()ing NULL
coverity: don't risk exec()ing NULL

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

akshaykarle/lxc,virajs/lxc,JohnPeacockMessageSystems/lxc,JohnPeacockMessageSystems/lxc,caglar10ur/lxc,LynxChaus/lxc,fwilson42/lxc,rowhit/lxc,major/lxc,cjwatson/lxc,QingweiPeterLan/cs188-lxc,Pelagicore/lxc,virajs/lxc,GreatFruitOmsk/lxc,bmoar/lxc,lazy404/lxc,armcc/lxc,lazy404/lxc,major/lxc,aeris/lxc,ioops/lxc,ss1h2a3tw/lxc,akshaykarle/lxc,raspberrypython/lxc,rldleblanc/lxc,bostjan/lxc,ksperis/lxc,JohnPeacockMessageSystems/lxc,terceiro/lxc,hfuCN/lxc,bmoar/lxc,christiaan/lxc,ysbnim/lxc,thmo/lxc,Distrotech/lxc,fwilson42/lxc,Ponce/lxc,terceiro/lxc,tych0/lxc,akshaykarle/lxc,ioops/lxc,ioops/lxc,GreatFruitOmsk/lxc,hfuCN/lxc,ksperis/lxc,rowhit/lxc,Ponce/lxc,christiaan/lxc,ksperis/lxc,lisongmin/lxc,lisongmin/lxc,codebauss/lxc,ysbnim/lxc,hfuCN/lxc,szaszg/lxc,yubo/lxc,codebauss/lxc,raspberrypython/lxc,federicobriata/lxc,terceiro/lxc,GreatFruitOmsk/lxc,hallyn/lxc,efiop/lxc,efiop/lxc,yubo/lxc,rowhit/lxc,armcc/lxc,caglar10ur/lxc,Ponce/lxc,aeris/lxc,DarknessBeforeDawn/lxc,hallyn/lxc,yubo/lxc,rldleblanc/lxc,ss1h2a3tw/lxc,lazy404/lxc,jirislaby/lxc,LynxChaus/lxc,federicobriata/lxc,thmo/lxc,ss1h2a3tw/lxc,sapun/lxc,christiaan/lxc,hallyn/lxc,jirislaby/lxc,bmoar/lxc,bostjan/lxc,raspberrypython/lxc,LynxChaus/lxc,virajs/lxc,major/lxc,Distrotech/lxc,fwilson42/lxc,ecarrara/lxc,szaszg/lxc,QingweiPeterLan/cs188-lxc,rldleblanc/lxc,ysbnim/lxc,QingweiPeterLan/cs188-lxc,armcc/lxc,federicobriata/lxc,aeris/lxc,ecarrara/lxc,cjwatson/lxc,efiop/lxc,cjwatson/lxc,ecarrara/lxc,jirislaby/lxc,DarknessBeforeDawn/lxc,tych0/lxc,Pelagicore/lxc,DarknessBeforeDawn/lxc,codebauss/lxc,szaszg/lxc,lisongmin/lxc,sapun/lxc,tych0/lxc,bostjan/lxc,Distrotech/lxc,thmo/lxc,Pelagicore/lxc,sapun/lxc,caglar10ur/lxc