Fix bug #65667: ftp_nb_continue produces segfault

the idea behind ftp_nb_get is for it to be followed by multiple calls
to ftp_nb_continue in order to download a file piece-by-piece.

As such, it's unwise to close the stream used to write the downloaded
data to when the file hasn't been completely downloaded within the first
call to ftp_nb_get.

This regression was added in a93a462dcefd62e07963dd2da506fbb3409c88b5
and this patch restores the behavior that was seen pre-patch.