fix core dump when using physical interface
fix core dump when using physical interface

If the physical link is not specified in the configuration
the check in if_nametoindex(netdev->link) leads to a segfault.

Check the link is specified.

Signed-off-by: Daniel Lezcano <3211518dd4d87cb3f7ee12cb52e14b7fb2bc0446@fr.ibm.com>
Reported-by: Ferenc Wagner <684c7697b684336a4b6853ff802984f147a84f41@niif.hu>

Pelagicore/lxc,major/lxc,glensc/lxc,hfuCN/lxc,akshaykarle/lxc,codebauss/lxc,federicobriata/lxc,ecarrara/lxc,codebauss/lxc,GreatFruitOmsk/lxc,hallyn/lxc,fwilson42/lxc,Ponce/lxc,rowhit/lxc,DarknessBeforeDawn/lxc,potatosalad/lxc,efiop/lxc,bmoar/lxc,Pelagicore/lxc,regit/lxc,armcc/lxc,terceiro/lxc,akshaykarle/lxc,cjwatson/lxc,hfuCN/lxc,ysbnim/lxc,ss1h2a3tw/lxc,szaszg/lxc,ioops/lxc,JohnPeacockMessageSystems/lxc,Ponce/lxc,hallyn/lxc,bostjan/lxc,rldleblanc/lxc,LynxChaus/lxc,LynxChaus/lxc,major/lxc,virajs/lxc,raspberrypython/lxc,aeris/lxc,ioops/lxc,christiaan/lxc,yubo/lxc,smoser/lxc,hkjolhede/lxc,ojkastl/lxc_Patches,GreatFruitOmsk/lxc,sapun/lxc,smoser/lxc,sapun/lxc,abstrakraft/lxc-android-lxc,ecarrara/lxc,jirislaby/lxc,ostap/lxc,federicobriata/lxc,inokappa/lxc,ojkastl/lxc_Patches,QingweiPeterLan/cs188-lxc,caglar10ur/lxc,lisongmin/lxc,yubo/lxc,usc-isi/lxc,rowhit/lxc,Ponce/lxc,ysbnim/lxc,lazy404/lxc,hkjolhede/lxc,QingweiPeterLan/cs188-lxc,JohnPeacockMessageSystems/lxc,ss1h2a3tw/lxc,smoser/lxc,JohnPeacockMessageSystems/lxc,cjwatson/lxc,coconutpilot/lxc-gentoo-new,aeris/lxc,inokappa/lxc,weizhenwei/lxc,DarknessBeforeDawn/lxc,Distrotech/lxc,efiop/lxc,ojkastl/lxc_Patches,ss1h2a3tw/lxc,lazy404/lxc,kylef/lxc,caglar10ur/lxc,DarknessBeforeDawn/lxc,sapun/lxc,rldleblanc/lxc,szaszg/lxc,thmo/lxc,cjwatson/lxc,bostjan/lxc,usc-isi/lxc,abstrakraft/lxc-android-lxc,terceiro/lxc,aeris/lxc,lazy404/lxc,ostap/lxc,ksperis/lxc,ysbnim/lxc,hfuCN/lxc,abstrakraft/lxc-android-lxc,hallyn/lxc,raspberrypython/lxc,lisongmin/lxc,weizhenwei/lxc,regit/lxc,smoser/lxc,Distrotech/lxc,hkjolhede/lxc,bmoar/lxc,QingweiPeterLan/cs188-lxc,abstrakraft/lxc-android-lxc,ojkastl/lxc_Patches,raspberrypython/lxc,efiop/lxc,bostjan/lxc,regit/lxc,terceiro/lxc,szaszg/lxc,armcc/lxc,Pelagicore/lxc,tych0/lxc,glensc/lxc,potatosalad/lxc,codebauss/lxc,jirislaby/lxc,federicobriata/lxc,coconutpilot/lxc-gentoo-new,major/lxc,armcc/lxc,ksperis/lxc,kylef/lxc,ioops/lxc,fwilson42/lxc,tych0/lxc,bmoar/lxc,christiaan/lxc,thmo/lxc,lisongmin/lxc,virajs/lxc,fwilson42/lxc,LynxChaus/lxc,rldleblanc/lxc,tych0/lxc,usc-isi/lxc,weizhenwei/lxc,caglar10ur/lxc,glensc/lxc,yubo/lxc,virajs/lxc,Distrotech/lxc,rowhit/lxc,akshaykarle/lxc,GreatFruitOmsk/lxc,ksperis/lxc,jirislaby/lxc,ecarrara/lxc,thmo/lxc,christiaan/lxc