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

tools/testing/selftests/bpf/prog_tests/cg_storage_multi.c

@@ -147,7 +147,9 @@ static void test_egress_only(int parent_cgroup_fd, int child_cgroup_fd)
 		goto close_bpf_object;
 
 close_bpf_object:
+	if (!IS_ERR(parent_link))
 		bpf_link__destroy(parent_link);
+	if (!IS_ERR(child_link))
 		bpf_link__destroy(child_link);
 
 	cg_storage_multi_egress_only__destroy(obj);
@@ -262,11 +264,17 @@ static void test_isolated(int parent_cgroup_fd, int child_cgroup_fd)
 		goto close_bpf_object;
 
 close_bpf_object:
+	if (!IS_ERR(parent_egress1_link))
 		bpf_link__destroy(parent_egress1_link);
+	if (!IS_ERR(parent_egress2_link))
 		bpf_link__destroy(parent_egress2_link);
+	if (!IS_ERR(parent_ingress_link))
 		bpf_link__destroy(parent_ingress_link);
+	if (!IS_ERR(child_egress1_link))
 		bpf_link__destroy(child_egress1_link);
+	if (!IS_ERR(child_egress2_link))
 		bpf_link__destroy(child_egress2_link);
+	if (!IS_ERR(child_ingress_link))
 		bpf_link__destroy(child_ingress_link);
 
 	cg_storage_multi_isolated__destroy(obj);
@@ -367,11 +375,17 @@ static void test_shared(int parent_cgroup_fd, int child_cgroup_fd)
 		goto close_bpf_object;
 
 close_bpf_object:
+	if (!IS_ERR(parent_egress1_link))
 		bpf_link__destroy(parent_egress1_link);
+	if (!IS_ERR(parent_egress2_link))
 		bpf_link__destroy(parent_egress2_link);
+	if (!IS_ERR(parent_ingress_link))
 		bpf_link__destroy(parent_ingress_link);
+	if (!IS_ERR(child_egress1_link))
 		bpf_link__destroy(child_egress1_link);
+	if (!IS_ERR(child_egress2_link))
 		bpf_link__destroy(child_egress2_link);
+	if (!IS_ERR(child_ingress_link))
 		bpf_link__destroy(child_ingress_link);
 
 	cg_storage_multi_shared__destroy(obj);