libbpf: Skip BTF fixup if object file has no BTF

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

Fixes: 8fd27bf ("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

tools/lib/bpf/linker.c

@@ -1313,6 +1313,9 @@ static int linker_fixup_btf(struct src_obj *obj)
 	struct src_sec *sec;
 	int i, j, n, m;
 
+	if (!obj->btf)
+		return 0;
+
 	n = btf__get_nr_types(obj->btf);
 	for (i = 1; i <= n; i++) {
 		struct btf_var_secinfo *vi;