storage: avoid null deref on qemu-img failure
storage: avoid null deref on qemu-img failure

Detected by Coverity.  Only possible if qemu-img gives bogus output,
but we might as well be robust.

* src/storage/storage_backend.c
(virStorageBackendQEMUImgBackingFormat): Check for strstr failure.

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