x86/ioperm: Initialize pointer bitmap with NULL rather than 0

The pointer bitmap is being initialized with a plain integer 0,
fix this by initializing it with a NULL instead.

Cleans up sparse warning:
arch/x86/xen/enlighten_pv.c:876:27: warning: Using plain integer
as NULL pointer

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200721100217.407975-1-colin.king@canonical.com