libbpf: Fix error path in bpf_object__elf_init()

When it failed to get section names, it should call into
bpf_object__elf_finish() like others.

Fixes: 88a82120 ("libbpf: Factor out common ELF operations and improve logging")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210317145414.884817-1-namhyung@kernel.org