ext4: fix potential null dereference in ext4 kunit test

ext4: fix potential null dereference in ext4 kunit test

kunit_kzalloc() may return a NULL pointer, dereferencing it
without NULL check may lead to NULL dereference.
Add a NULL check for grp.

Fixes: ac96b56a2fbd ("ext4: Add unit test for mb_mark_used")
Fixes: b7098e1fa7bc ("ext4: Add unit test for mb_free_blocks")
Signed-off-by: Charles Han <hanchunchao@inspur.com>
Reviewed-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://patch.msgid.link/20250110092421.35619-1-hanchunchao@inspur.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>