ext4: avoid ptr null pointer dereference

When commit 13df4d44 ("ext4: fix slab-out-of-bounds in
ext4_mb_find_good_group_avg_frag_lists()") was backported to stable, the
commit f536808a ("ext4: refactor out ext4_generic_attr_store()") that
uniformly determines if the ptr is null is not merged in, so it needs to
be judged whether ptr is null or not in each case of the switch, otherwise
null pointer dereferencing may occur.

Fixes: 677ff458 ("ext4: fix slab-out-of-bounds in ext4_mb_find_good_group_avg_frag_lists()")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>