Semantic fix in the X11 `authentication failed' error packet

Semantic fix in the X11 `authentication failed' error packet
construction. Doesn't actually affect anything right now, since the
bug was a failure to round a length up to the next multiple of 4 and
it so happens that our current message was exactly 40 bytes anyway
:-) But if we start giving a wider variety of messages one day then
it might be handy to be able to do them without gratuitous crashes.

git-svn-id: svn://svn.tartarus.org/sgt/putty@1222 cda61777-01e9-0310-a592-d414129be87e