tests: avoid null pointer dereference
tests: avoid null pointer dereference

Unlikely to hit in real life, but clang noticed it.

* tests/commandtest.c (checkoutput, test4, test18): Avoid
unlink(NULL) on OOM.

rmarwaha/libvirt1,danwent/libvirt-ovs,andreabolognani/libvirt,nertpinx/libvirt,novel/fbsd-libvirt,iam-TJ/libvirt,taget/libvirt,datto/libvirt,nertpinx/libvirt,leilihh/libvirt,novel/fbsd-libvirt,datto/libvirt,jeckersb/libvirt,iam-TJ/libvirt,novel/fbsd-libvirt,siboulet/libvirt-openvz,rmarwaha/libvirt1,wiedi/libvirt,zippy2/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,crobinso/libvirt,trainstack/libvirt,danwent/libvirt-ovs,rmarwaha/libvirt,rmarwaha/libvirt,warewolf/libvirt,taget/libvirt,agx/libvirt,jeckersb/libvirt,libvirt/libvirt,andreabolognani/libvirt,jfehlig/libvirt,jardasgit/libvirt,zippy2/libvirt,jeckersb/libvirt,elmarco/libvirt,warewolf/libvirt,andreabolognani/libvirt,shugaoye/libvirt,novel/fbsd-libvirt,usc-isi/libvirt,rlaager/libvirt,zhlcindy/libvirt-1.1.4-maintain,wiedi/libvirt,dumbbell/libvirt,agx/libvirt,leilihh/libvirt,jfehlig/libvirt,novel/fbsd-libvirt,jeckersb/libvirt,trainstack/libvirt,shugaoye/libvirt,iam-TJ/libvirt,trainstack/libvirt,jfehlig/libvirt,emaste/libvirt,crobinso/libvirt,bjzhang/libvirt,rmarwaha/libvirt1,fabianfreyer/libvirt,warewolf/libvirt,rlaager/libvirt,eskultety/libvirt,rlaager/libvirt,siboulet/libvirt-openvz,siboulet/libvirt-openvz,libvirt/libvirt,dumbbell/libvirt,foomango/libvirt,bjzhang/libvirt,trainstack/libvirt,VenkatDatta/libvirt,foomango/libvirt,shugaoye/libvirt,warewolf/libvirt,foomango/libvirt,foomango/libvirt,taget/libvirt,VenkatDatta/libvirt,dumbbell/libvirt,zhlcindy/libvirt-1.1.4-maintain,zippy2/libvirt,eskultety/libvirt,agx/libvirt,crobinso/libvirt,andreabolognani/libvirt,wiedi/libvirt,rlaager/libvirt,cbosdo/libvirt,kantai/libvirt-vfork,dumbbell/libvirt,elmarco/libvirt,rmarwaha/libvirt,danwent/libvirt-ovs,novel/fbsd-libvirt,leilihh/libvirt,warewolf/libvirt,jeckersb/libvirt,shugaoye/libvirt,VenkatDatta/libvirt,olafhering/libvirt,iam-TJ/libvirt,soulxu/libvirt-xuhj,bjzhang/libvirt,rmarwaha/libvirt1,iam-TJ/libvirt,wiedi/libvirt,rmarwaha/libvirt,soulxu/libvirt-xuhj,jardasgit/libvirt,olafhering/libvirt,soulxu/libvirt-xuhj,novel/fbsd-libvirt,emaste/libvirt,cbosdo/libvirt,usc-isi/libvirt,agx/libvirt,cbosdo/libvirt,olafhering/libvirt,zhlcindy/libvirt-1.1.4-maintain,datto/libvirt,cbosdo/libvirt,kantai/libvirt-vfork,novel/fbsd-libvirt,taget/libvirt,novel/fbsd-libvirt,kantai/libvirt-vfork,trainstack/libvirt,taget/libvirt,jardasgit/libvirt,rmarwaha/libvirt,bjzhang/libvirt,wiedi/libvirt,eskultety/libvirt,olafhering/libvirt,iam-TJ/libvirt,dumbbell/libvirt,datto/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,crobinso/libvirt,trainstack/libvirt,wiedi/libvirt,warewolf/libvirt,elmarco/libvirt,trainstack/libvirt,andreabolognani/libvirt,leilihh/libvirt,emaste/libvirt,jeckersb/libvirt,eskultety/libvirt,usc-isi/libvirt,kantai/libvirt-vfork,elmarco/libvirt,kantai/libvirt-vfork,rmarwaha/libvirt1,agx/libvirt,soulxu/libvirt-xuhj,bjzhang/libvirt,jeckersb/libvirt,jardasgit/libvirt,libvirt/libvirt,emaste/libvirt,emaste/libvirt,fabianfreyer/libvirt,usc-isi/libvirt,eskultety/libvirt,rmarwaha/libvirt1,danwent/libvirt-ovs,libvirt/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,foomango/libvirt,warewolf/libvirt,usc-isi/libvirt,rlaager/libvirt,rmarwaha/libvirt,zhlcindy/libvirt-1.1.4-maintain,jfehlig/libvirt,zhlcindy/libvirt-1.1.4-maintain,wiedi/libvirt,leilihh/libvirt,siboulet/libvirt-openvz,VenkatDatta/libvirt,nertpinx/libvirt,emaste/libvirt,zippy2/libvirt,emaste/libvirt,dumbbell/libvirt,elmarco/libvirt,iam-TJ/libvirt,danwent/libvirt-ovs,leilihh/libvirt,shugaoye/libvirt,siboulet/libvirt-openvz,nertpinx/libvirt,soulxu/libvirt-xuhj,datto/libvirt,cbosdo/libvirt