Btrfs: avoid possible NULL deref in io_ctl_drop_pages()

If we run into some failure path in io_ctl_prepare_pages(),
io_ctl->pages[] array may have some NULL pointers.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

fs/btrfs/free-space-cache.c

@@ -319,10 +319,12 @@ static void io_ctl_drop_pages(struct io_ctl *io_ctl)
 	io_ctl_unmap_page(io_ctl);
 
 	for (i = 0; i < io_ctl->num_pages; i++) {
+		if (io_ctl->pages[i]) {
 			ClearPageChecked(io_ctl->pages[i]);
 			unlock_page(io_ctl->pages[i]);
 			page_cache_release(io_ctl->pages[i]);
 		}
+	}
 }
 
 static int io_ctl_prepare_pages(struct io_ctl *io_ctl, struct inode *inode,