f2fs: check curseg->inited before write_sum_page in change_curseg

In the __f2fs_init_atgc_curseg->get_atssr_segment calling,
curseg->segno is NULL_SEGNO, indicating that there is no summary
block that needs to be written.

Fixes: 093749e2 ("f2fs: support age threshold based garbage collection")
Signed-off-by: Yongpeng Yang <yangyongpeng1@oppo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>