libbpf: Skip BTF fixup if object file has no BTF

Skip BTF fixup step when input object file is missing BTF altogether.

Fixes: 8fd27bf6 ("libbpf: Add BPF static linker BTF and BTF.ext support")
Reported-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/bpf/20210319205909.1748642-3-andrii@kernel.org