ntfs: fix variable dereferenced before check warnings

ntfs: fix variable dereferenced before check warnings

Detected by Smatch.

lcnalloc.c:736 ntfs_cluster_alloc() error:
  we previously assumed 'rl' could be null (see line 719)

inode.c:3275 ntfs_inode_close() warn:
  variable dereferenced before check 'tmp_nis' (see line 3255)

attrib.c:4952 ntfs_attr_remove() warn:
  variable dereferenced before check 'ni' (see line 4951)

dir.c:1035 ntfs_readdir() error:
  we previously assumed 'private' could be null (see line 850)

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>