libbpf: Check bpf_map_skeleton link for NULL

Avoid dereferencing bpf_map_skeleton's link field if it's NULL.
If BPF map skeleton is created with the size, that indicates containing
link field, but the field was not actually initialized with valid
bpf_link pointer, libbpf crashes. This may happen when using libbpf-rs
skeleton.
Skeleton loading may still progress, but user needs to attach struct_ops
map separately.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250514113220.219095-1-mykyta.yatsenko5@gmail.com

Author: mykyta5

tools/lib/bpf/libbpf.c

@@ -14102,6 +14102,12 @@ int bpf_object__attach_skeleton(struct bpf_object_skeleton *s)
 		}
 
 		link = map_skel->link;
+		if (!link) {
+			pr_warn("map '%s': BPF map skeleton link is uninitialized\n",
+				bpf_map__name(map));
+			continue;
+		}
+
 		if (*link)
 			continue;