Fix an invalid strlen comparison in ssh_message_auth_reply_default

During the transition to strncat in ssh_message_auth_reply_default,
an invalid strlen comparison was added which causes the function
to fail whenever it's called.

Signed-off-by: Andrew Collins <bsderandrew@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/server.c

@@ -596,7 +596,7 @@ static int ssh_message_auth_reply_default(ssh_message msg,int partial) {
             sizeof(methods_c) - strlen(methods_c) - 1);
   }
 
-  if (methods_c[0] == '\0' || strlen(methods_c) != ',') {
+  if (methods_c[0] == '\0' || methods_c[strlen(methods_c)-1] != ',') {
       return SSH_ERROR;
   }