When a reused connection did transfer 0 bytes, it assumed the transfer
had failed and needed a retry. Add a check for data->red.done, so we can
successfully accept the transfer of a 0-length file via SFTP.
Add test case 1583 to verfiy.
Fix SFTP disconnect debug trace when there was nothing to
disconnect (like when reusing a connection).
Fixes#19165
Reported-by: Alexander Blach
Closes#19189
