fs/ntfs3: Fix NULL dereference in ni_write_inode

Syzbot reports a NULL dereference in ni_write_inode.
When creating a new inode, if allocation fails in mi_init function
(called in mi_format_new function), mi->mrec is set to NULL.
In the error path of this inode creation, mi->mrec is later
dereferenced in ni_write_inode.

Add a NULL check to prevent NULL dereference.

Link: https://syzkaller.appspot.com/bug?extid=f45957555ed4a808cc7a
Reported-and-tested-by:  <syzbot+f45957555ed4a808cc7a@syzkaller.appspotmail.com>
Signed-off-by: Abdun Nihaal <abdun.nihaal@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>