cifs: If a corrupted DACL is returned by the server, bail out.

Static code analysis reported a possible null pointer dereference
in my last commit:
cifs: Retain old ACEs when converting between mode bits and ACL.

This could happen if the DACL returned by the server is corrupted.
We were trying to continue by assuming that the file has empty DACL.
We should bail out with an error instead.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reported-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Rohith Surabattula <rohiths@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>