smb/server: fix potential null-ptr-deref of lease_ctx_info in smb2_open()

null-ptr-deref will occur when (req_op_level == SMB2_OPLOCK_LEVEL_LEASE)
and parse_lease_state() return NULL.

Fix this by check if 'lease_ctx_info' is NULL.

Additionally, remove the redundant parentheses in
parse_durable_handle_context().

Signed-off-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Signed-off-by: Steve French <stfrench@microsoft.com>