GH-123: Fix free of an uninitialised pointer.

CHAN_AGENT channels need c->u.a.message to be either NULL or valid
dynamically allocated memory, because it'll be freed by
ssh_channel_destroy. This bug triggers if an agent forwarding channel
is opened and closed without having sent any queries.

git-svn-id: svn://svn.tartarus.org/sgt/putty@10032 cda61777-01e9-0310-a592-d414129be87e