f2fs: fix to verify tpage before releasing in f2fs_free_dic()

In below error path, tpages[i] could be NULL, fix to check it before
releasing it.
- f2fs_read_multi_pages
 - f2fs_alloc_dic
  - f2fs_free_dic

Fixes: 61fbae2b ("f2fs: fix to avoid NULL pointer dereference")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/compress.c

@@ -1333,6 +1333,8 @@ void f2fs_free_dic(struct decompress_io_ctx *dic)
 		for (i = 0; i < dic->cluster_size; i++) {
 			if (dic->rpages[i])
 				continue;
+			if (!dic->tpages[i])
+				continue;
 			unlock_page(dic->tpages[i]);
 			put_page(dic->tpages[i]);
 		}