libbpf: return error when if_nametoindex returns 0
libbpf: return error when if_nametoindex returns 0

sll_ifindex=0 matches any interface, which is not exepected here

brendangregg/bcc,tuxology/bcc,iovisor/bcc,tuxology/bcc,iovisor/bcc,mcaleavya/bcc,brendangregg/bcc,tuxology/bcc,mcaleavya/bcc,tuxology/bcc,tuxology/bcc,brendangregg/bcc,iovisor/bcc,mcaleavya/bcc,iovisor/bcc,brendangregg/bcc,mcaleavya/bcc,iovisor/bcc,mcaleavya/bcc,brendangregg/bcc