fs: afs: Fix a possible null-pointer dereference in afs_put_read()

In afs_read_dir(), there is an if statement on line 255 to check whether
req->pages is NULL:
	if (!req->pages)
		goto error;

If req->pages is NULL, afs_put_read() on line 337 is executed.
In afs_put_read(), req->pages[i] is used on line 195.
Thus, a possible null-pointer dereference may occur in this case.

To fix this possible bug, an if statement is added in afs_put_read() to
check req->pages.

This bug is found by a static analysis tool STCheck written by us.

Fixes: f3ddee8d ("afs: Fix directory handling")
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/file.c

@@ -191,11 +191,13 @@ void afs_put_read(struct afs_read *req)
 	int i;
 
 	if (refcount_dec_and_test(&req->usage)) {
+		if (req->pages) {
 			for (i = 0; i < req->nr_pages; i++)
 				if (req->pages[i])
 					put_page(req->pages[i]);
 			if (req->pages != req->array)
 				kfree(req->pages);
+		}
 		kfree(req);
 	}
 }