security_dac: Avoid segfault when no label is requested
security_dac: Avoid segfault when no label is requested

When no DAC "label" was requested for a domain the DAC manager tried to
strdup a NULL string causing a segfault.

cbosdo/libvirt,andreabolognani/libvirt,bjzhang/libvirt,olafhering/libvirt,agx/libvirt,andreabolognani/libvirt,cbosdo/libvirt,novel/fbsd-libvirt,andreabolognani/libvirt,agx/libvirt,wiedi/libvirt,rlaager/libvirt,bjzhang/libvirt,siboulet/libvirt-openvz,zhlcindy/libvirt-1.1.4-maintain,elmarco/libvirt,iam-TJ/libvirt,novel/fbsd-libvirt,jardasgit/libvirt,elmarco/libvirt,emaste/libvirt,wiedi/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,bjzhang/libvirt,emaste/libvirt,rlaager/libvirt,shugaoye/libvirt,siboulet/libvirt-openvz,emaste/libvirt,agx/libvirt,taget/libvirt,fabianfreyer/libvirt,novel/fbsd-libvirt,warewolf/libvirt,eskultety/libvirt,jfehlig/libvirt,datto/libvirt,jfehlig/libvirt,shugaoye/libvirt,warewolf/libvirt,olafhering/libvirt,VenkatDatta/libvirt,emaste/libvirt,bjzhang/libvirt,nertpinx/libvirt,warewolf/libvirt,novel/fbsd-libvirt,wiedi/libvirt,VenkatDatta/libvirt,emaste/libvirt,wiedi/libvirt,libvirt/libvirt,jeckersb/libvirt,eskultety/libvirt,novel/fbsd-libvirt,zhlcindy/libvirt-1.1.4-maintain,VenkatDatta/libvirt,jeckersb/libvirt,siboulet/libvirt-openvz,emaste/libvirt,trainstack/libvirt,datto/libvirt,nertpinx/libvirt,iam-TJ/libvirt,cbosdo/libvirt,rlaager/libvirt,libvirt/libvirt,taget/libvirt,rlaager/libvirt,warewolf/libvirt,rlaager/libvirt,jeckersb/libvirt,zhlcindy/libvirt-1.1.4-maintain,iam-TJ/libvirt,shugaoye/libvirt,agx/libvirt,wiedi/libvirt,jfehlig/libvirt,jardasgit/libvirt,crobinso/libvirt,iam-TJ/libvirt,nertpinx/libvirt,wiedi/libvirt,zippy2/libvirt,zhlcindy/libvirt-1.1.4-maintain,andreabolognani/libvirt,datto/libvirt,taget/libvirt,zhlcindy/libvirt-1.1.4-maintain,novel/fbsd-libvirt,fabianfreyer/libvirt,trainstack/libvirt,trainstack/libvirt,cbosdo/libvirt,fabianfreyer/libvirt,libvirt/libvirt,novel/fbsd-libvirt,iam-TJ/libvirt,novel/fbsd-libvirt,zippy2/libvirt,nertpinx/libvirt,siboulet/libvirt-openvz,agx/libvirt,fabianfreyer/libvirt,emaste/libvirt,zippy2/libvirt,iam-TJ/libvirt,warewolf/libvirt,jardasgit/libvirt,taget/libvirt,jeckersb/libvirt,eskultety/libvirt,taget/libvirt,VenkatDatta/libvirt,eskultety/libvirt,datto/libvirt,iam-TJ/libvirt,datto/libvirt,jardasgit/libvirt,trainstack/libvirt,jeckersb/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,elmarco/libvirt,warewolf/libvirt,jeckersb/libvirt,olafhering/libvirt,jardasgit/libvirt,shugaoye/libvirt,elmarco/libvirt,libvirt/libvirt,trainstack/libvirt,crobinso/libvirt,elmarco/libvirt,trainstack/libvirt,fabianfreyer/libvirt,trainstack/libvirt,crobinso/libvirt,siboulet/libvirt-openvz,eskultety/libvirt,jfehlig/libvirt,cbosdo/libvirt,novel/fbsd-libvirt,jeckersb/libvirt,shugaoye/libvirt,wiedi/libvirt,bjzhang/libvirt,warewolf/libvirt,crobinso/libvirt