selftests/bpf: Fix invalid pointer check in get_xlated_program()

Dan Carpenter reported invalid check for calloc() result in
test_verifier.c:get_xlated_program():

  ./tools/testing/selftests/bpf/test_verifier.c:1365 get_xlated_program()
  warn: variable dereferenced before check 'buf' (see line 1364)

  ./tools/testing/selftests/bpf/test_verifier.c
    1363		*cnt = xlated_prog_len / buf_element_size;
    1364		*buf = calloc(*cnt, buf_element_size);
    1365		if (!buf) {

  This should be if (!*buf) {

    1366			perror("can't allocate xlated program buffer");
    1367			return -ENOMEM;

This commit refactors the get_xlated_program() to avoid using double
pointer type.

Fixes: 933ff531 ("selftests/bpf: specify expected instructions in test_verifier tests")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Closes: https://lore.kernel.org/bpf/ZH7u0hEGVB4MjGZq@moroto/
Link: https://lore.kernel.org/bpf/20230609221637.2631800-1-eddyz87@gmail.com