libio: Use NULL instead of 0 as a null pointer constant

libio: Use NULL instead of 0 as a null pointer constant

This was missed in a recent global change.

Fixes: 53fcdf5f743a (2024-11-25, "Silence most -Wzero-as-null-pointer-constant diagnostics")
Reported-by: "Maciej W. Rozycki" <macro@redhat.com>
Cc: Siddhesh Poyarekar <siddhesh@sourceware.org>
Cc: Bruno Haible <bruno@clisp.org>
Cc: Martin Uecker <uecker@tugraz.at>
Cc: Xi Ruoyao <xry111@xry111.site>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Joseph Myers <josmyers@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Maciej W. Rozycki <macro@redhat.com>