net: leaseshelper: Don't crash if DNSMASQ doesn't provide lease expiry
net: leaseshelper: Don't crash if DNSMASQ doesn't provide lease expiry

The value is provided via environment and causes a crash if not defined.

rlaager/libvirt,agx/libvirt,eskultety/libvirt,elmarco/libvirt,VenkatDatta/libvirt,agx/libvirt,libvirt/libvirt,zippy2/libvirt,jardasgit/libvirt,zippy2/libvirt,agx/libvirt,crobinso/libvirt,datto/libvirt,andreabolognani/libvirt,shugaoye/libvirt,libvirt/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,olafhering/libvirt,VenkatDatta/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,elmarco/libvirt,jardasgit/libvirt,shugaoye/libvirt,elmarco/libvirt,taget/libvirt,jfehlig/libvirt,datto/libvirt,shugaoye/libvirt,shugaoye/libvirt,crobinso/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,olafhering/libvirt,eskultety/libvirt,andreabolognani/libvirt,rlaager/libvirt,andreabolognani/libvirt,olafhering/libvirt,fabianfreyer/libvirt,taget/libvirt,taget/libvirt,datto/libvirt,rlaager/libvirt,elmarco/libvirt,eskultety/libvirt,zippy2/libvirt,olafhering/libvirt,eskultety/libvirt,crobinso/libvirt,datto/libvirt,zippy2/libvirt,VenkatDatta/libvirt,taget/libvirt,rlaager/libvirt,shugaoye/libvirt,datto/libvirt,eskultety/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,rlaager/libvirt,elmarco/libvirt,nertpinx/libvirt,jardasgit/libvirt,nertpinx/libvirt,nertpinx/libvirt,agx/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,agx/libvirt,nertpinx/libvirt,jardasgit/libvirt,libvirt/libvirt,taget/libvirt