selftests/nolibc: stat_fault: silence NULL argument warning with glibc

Use another invalid address (void *)1 instead of NULL to silence this
compile warning with glibc:

    $ make libc-test
      CC      libc-test
    nolibc-test.c: In function ‘run_syscall’:
    nolibc-test.c:622:49: warning: null argument where non-null required (argument 1) [-Wnonnull]
      622 |   CASE_TEST(stat_fault);        EXPECT_SYSER(1, stat(NULL, &stat_buf), -1, EFAULT); break;
          |                                                 ^~~~
    nolibc-test.c:304:79: note: in definition of macro ‘EXPECT_SYSER2’
      304 |  do { if (!cond) pad_spc(llen, 64, "[SKIPPED]\n"); else ret += expect_syserr2(expr, expret, experr1, experr2, llen); } while (0)
          |                                                                               ^~~~
    nolibc-test.c:622:33: note: in expansion of macro ‘EXPECT_SYSER’
      622 |   CASE_TEST(stat_fault);        EXPECT_SYSER(1, stat(NULL, &stat_buf), -1, EFAULT); break;

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>