nwfilter: report an error on OOM
nwfilter: report an error on OOM

Also removed some unreachable code found by coverity:
libvirt-0.10.2/src/nwfilter/nwfilter_driver.c:259: unreachable: This
code cannot be reached: "nwfilterDriverUnlock(driver...".

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