ext4: correct grp validation in ext4_mb_good_group

Group corruption check will access memory of grp and will trigger kernel
crash if grp is NULL. So do NULL check before corruption check.

Fixes: 5354b2af ("ext4: allow ext4_get_group_info() to fail")
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20230801143204.2284343-2-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>