tools/nolibc: avoid using plain integer as NULL pointer

tools/nolibc: avoid using plain integer as NULL pointer

While an integer zero is a valid NULL pointer as per the C standard,
sparse will complain about it.

Use explicit NULL pointers instead.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202509261452.g5peaXCc-lkp@intel.com/
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>