lkdtm: cfi: use NULL for a null pointer rather than zero

There is a pointer being initialized with a zero, use NULL instead.

Cleans up sparse warning:
drivers/misc/lkdtm/cfi.c:100:27: warning: Using plain integer as NULL pointer
Acked-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NColin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220612202708.2754270-1-colin.i.king@gmail.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm/cfi.c

@@ -97,7 +97,7 @@ static volatile int force_check;
 static void lkdtm_CFI_BACKWARD(void)
 {
 	/* Use calculated gotos to keep labels addressable. */
-	void *labels[] = {0, &&normal, &&redirected, &&check_normal, &&check_redirected};
+	void *labels[] = { NULL, &&normal, &&redirected, &&check_normal, &&check_redirected };
 
 	pr_info("Attempting unchecked stack return address redirection ...\n");