agp: use NULL instead of 0 when pointer is needed

Fixes sparse warning:

drivers/char/agp/generic.c:1217:33: warning: Using plain integer as
NULL pointer
Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
CC: linux-kernel@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/char/agp/generic.c

@@ -1214,7 +1214,7 @@ struct agp_memory *agp_generic_alloc_user(size_t page_count, int type)
 		return NULL;
 
 	for (i = 0; i < page_count; i++)
-		new->pages[i] = 0;
+		new->pages[i] = NULL;
 	new->page_count = 0;
 	new->type = type;
 	new->num_scratch_pages = pages;