selftests/bpf: fix segfault of test_progs when prog loading failed

The test_progs subtests, test_spin_lock() and test_map_lock(),
requires BTF present to run successfully.
Currently, when BTF failed to load, test_progs will segfault,
  $ ./test_progs
  ...
  12: (bf) r1 = r8
  13: (85) call bpf_spin_lock#93
  map 'hash_map' has to have BTF in order to use bpf_spin_lock

  libbpf: -- END LOG --
  libbpf: failed to load program 'map_lock_demo'
  libbpf: failed to load object './test_map_lock.o'
  test_map_lock:bpf_prog_load errno 13
  Segmentation fault

The segfault is caused by uninitialized variable "obj", which
is used in bpf_object__close(obj), when bpf prog failed to load.

Initializing variable "obj" to NULL in two occasions fixed the problem.
  $ ./test_progs
  ...
  Summary: 219 PASSED, 2 FAILED

Fixes: b4d4556c ("selftests/bpf: add bpf_spin_lock verifier tests")
Fixes: ba72a7b4 ("selftests/bpf: test for BPF_F_LOCK")
Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

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

@@ -37,7 +37,7 @@ void test_map_lock(void)
 	const char *file = "./test_map_lock.o";
 	int prog_fd, map_fd[2], vars[17] = {};
 	pthread_t thread_id[6];
-	struct bpf_object *obj;
+	struct bpf_object *obj = NULL;
 	int err = 0, key = 0, i;
 	void *ret;
 

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

@@ -5,7 +5,7 @@ void test_spinlock(void)
 {
 	const char *file = "./test_spin_lock.o";
 	pthread_t thread_id[4];
-	struct bpf_object *obj;
+	struct bpf_object *obj = NULL;
 	int prog_fd;
 	int err = 0, i;
 	void *ret;