ftp_do_more: consider DO_MORE complete when server connects back

In the case of an active connection when ftp_do_more() detects that the
server has connected back, it must make sure to mark it as complete so
that the multi_runsingle() function will detect this and move on to the
next state.

Bug: http://curl.haxx.se/mail/lib-2013-07/0115.html
Reported-by: Clemens Gruber