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: Kees Cook <[e-mail]>
Signed-off-by: Colin Ian King <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/lkdtm/cfi.c

···
 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");