tools: libbpf: handle NULL program gracefully in bpf_program__nth_fd()

bpf_map__fd() handles NULL map gracefully and returns -EINVAL.
bpf_program__fd() and bpf_program__nth_fd() crash in this case.
Make the behaviour more consistent by validating prog pointer
as well.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -1991,6 +1991,9 @@ int bpf_program__nth_fd(struct bpf_program *prog, int n)
 {
 	int fd;
 
+	if (!prog)
+		return -EINVAL;
+
 	if (n >= prog->instances.nr || n < 0) {
 		pr_warning("Can't get the %dth fd from program %s: only %d instances\n",
 			   n, prog->section_name, prog->instances.nr);