libbpf: Fix segfault in light skeleton for objects without BTF

When fed an empty BPF object, bpftool gen skeleton -L crashes at
btf__set_fd() since it assumes presence of obj->btf, however for
the sequence below clang adds no .BTF section (hence no BTF).

Reproducer:

$ touch a.bpf.c
$ clang -O2 -g -target bpf -c a.bpf.c
$ bpftool gen skeleton -L a.bpf.o
/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
/* THIS FILE IS AUTOGENERATED! */

struct a_bpf {
struct bpf_loader_ctx ctx;
Segmentation fault (core dumped)

The same occurs for files compiled without BTF info, i.e. without
clang's -g flag.

Fixes: 67234743736a (libbpf: Generate loader program out of BPF ELF file.)
Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/libbpf.c

···
 
 	if (obj->gen_loader) {
 		/* reset FDs */
-		btf__set_fd(obj->btf, -1);
+		if (obj->btf)
+			btf__set_fd(obj->btf, -1);
 		for (i = 0; i < obj->nr_maps; i++)
 			obj->maps[i].fd = -1;
 		if (!err)