If connecting to a server with an illegal userid und an rsa1 key, the connection
is dropped immediatly. Normaly, a wrong username will stay in the auth_loop and
keep requesting authentication from the user. The reason is in auth-rsa.c in
line 293:

         /* no user given */
         if (pw == NULL)
               return 0;

Invalid users nolonger are passwd as NULL-pointers, a faked pw struct is used
instead. This results in a call to temporarily_use_uid(pw) with the faked pw
struct, which fails with fatal().

As a patch I tried 

         /* no user given */
         if (pw == fakepw())
                 return 0;
which worked, but maybe is not the best solution.