ftp: fix write when out file is closed and emit condition.

* protect against write() call on a closed incoming stream.
* Use DataTransferEventCode::closed_by_peer when
data are not completly received, else ataTransferEventCode::finished
is emitted.

Change-Id: I15f2518f70afbe1bfe8aa8813cbca676dec4360a
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>