multi: start ftp state machine when switching to DO_MORE

This extends the fix from commit d7934b8b

When the multi state is changed within the multi_runsingle from DOING to
DO_MORE, we didn't immediately start the FTP state machine again. That
then left the FTP state in FTP_STOP. When curl_multi_fdset() was
subsequently called, the ftp_domore_getsock() function would return the
wrong fd info.

Reported by: Gokhan Sengun