ssh_sftp_do_select: don't fail if a callback is pending.

ssh_sftp_loop_iteration() used to return failure if no file handle was
in use for the select loop, on the basis that that means select would
just loop forever. But if there's a toplevel callback pending - in
particular, if it's going to do something like emptying ssh->in_raw
which will put an fd _back into_ the next iteration of the select loop
- then that's not a good enough reason to return permanent failure.
Just go round the loop, run the callback, and try again.

unix/uxsftp.c

@@ -463,7 +463,7 @@ static int ssh_sftp_do_select(bool include_stdin, bool no_fds_ok)
 	for (fd = first_fd(&fdstate, &rwx); fd >= 0;
 	     fd = next_fd(&fdstate, &rwx)) i++;
 
-	if (i < 1 && !no_fds_ok)
+	if (i < 1 && !no_fds_ok && !toplevel_callback_pending())
 	    return -1;		       /* doom */
 
 	/* Expand the fdlist buffer if necessary. */