selftests/nolibc: libc-test: avoid -Wstringop-overflow warnings

selftests/nolibc: libc-test: avoid -Wstringop-overflow warnings

Newer versions of glibc annotate the poll() function with
__attribute__(access) which triggers a compiler warning inside the
testcase poll_fault.
Avoid this by using a plain NULL which is enough for the testcase.
To avoid potential future warnings also adapt the other EFAULT
testcases, except select_fault as NULL is a valid value for its
argument.

nolibc-test.c: In function ‘run_syscall’:
nolibc-test.c:338:62: warning: ‘poll’ writing 8 bytes into a region of size 0 overflows the destination [-Wstringop-overflow=]
  338 |         do { if (!(cond)) result(llen, SKIPPED); else ret += expect_syserr2(expr, expret, experr1, experr2, llen); } while (0)
      |                                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
nolibc-test.c:341:9: note: in expansion of macro ‘EXPECT_SYSER2’
  341 |         EXPECT_SYSER2(cond, expr, expret, experr, 0)
      |         ^~~~~~~~~~~~~
nolibc-test.c:905:47: note: in expansion of macro ‘EXPECT_SYSER’
  905 |                 CASE_TEST(poll_fault);        EXPECT_SYSER(1, poll((void *)1, 1, 0), -1, EFAULT); break;
      |                                               ^~~~~~~~~~~~
cc1: note: destination object is likely at address zero
In file included from /usr/include/poll.h:1,
                 from nolibc-test.c:33:
/usr/include/sys/poll.h:54:12: note: in a call to function ‘poll’ declared with attribute ‘access (write_only, 1, 2)’
   54 | extern int poll (struct pollfd *__fds, nfds_t __nfds, int __timeout)
      |            ^~~~

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>