selftests/bpf: Don't destroy failed link

Check that link is NULL or proper pointer before invoking bpf_link__destroy().
Not doing this causes crash in test_progs, when cg_storage_multi selftest
fails.

Fixes: 3573f384 ("selftests/bpf: Test CGROUP_STORAGE behavior on shared egress + ingress")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200729045056.3363921-1-andriin@fb.com