Automatically delete the (partially downloaded / zero-byte) .QWK packet if

FTP_GET indicates download failure.

exec/qnet-ftp.src

@@ -178,6 +178,7 @@ if_true
 	lprintf "QNET-FTP: Downloaded %s successfully" str
 else
 	lprintf "QNET-FTP: %s NOT downloaded socket_error=%d (no new messages?)" str _socket_error
+    remove_file qwk
 	end_if
 
 #################