Fix FTPS upload failure: !DATA ERROR 0 receiving on data socket

Don't treat CRYPT_ERROR_COMPLETE (-24) as a socket error during upload since it's an indication that the remote closed the connection and is the normal "end of file/transfer" indicator, not an error. 'rd' is already 0 in this case, so no need to set at all (since recv() returns 0 upon disconnect and that's what we're emulating here).

Fixes issue #309 reported by Jas Hud.

src/sbbs3/ftpsrvr.c

@@ -978,7 +978,8 @@ static void receive_thread(void* arg)
 			int status = cryptPopData(*xfer.data_sess, buf, sizeof(buf), &rd);
 			if (status != CRYPT_OK) {
 				GCES(status, *xfer.data_sock, *xfer.data_sess, estr, "popping data");
-				rd = -1;
+				if (status != CRYPT_ERROR_COMPLETE)
+					rd = SOCKET_ERROR;
 			}
 		}
 		else {