libbpf: Fix possible NULL pointer dereference when destroying skeleton

When I checked the code in skeleton header file generated with my own
bpf prog, I found there may be possible NULL pointer dereference when
destroying skeleton. Then I checked the in-tree bpf progs, finding that is
a common issue. Let's take the generated samples/bpf/xdp_redirect_cpu.skel.h
for example. Below is the generated code in
xdp_redirect_cpu__create_skeleton():

	xdp_redirect_cpu__create_skeleton
		struct bpf_object_skeleton *s;
		s = (struct bpf_object_skeleton *)calloc(1, sizeof(*s));
		if (!s)
			goto error;
		...
	error:
		bpf_object__destroy_skeleton(s);
		return  -ENOMEM;

After goto error, the NULL 's' will be deferenced in
bpf_object__destroy_skeleton().

We can simply fix this issue by just adding a NULL check in
bpf_object__destroy_skeleton().

Fixes: d66562fb ("libbpf: Add BPF object skeleton support")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220108134739.32541-1-laoar.shao@gmail.com

tools/lib/bpf/libbpf.c

@@ -11795,6 +11795,9 @@ void bpf_object__detach_skeleton(struct bpf_object_skeleton *s)
 
 void bpf_object__destroy_skeleton(struct bpf_object_skeleton *s)
 {
+	if (!s)
+		return;
+
 	if (s->progs)
 		bpf_object__detach_skeleton(s);
 	if (s->obj)