Fixed inactive xfers may be disconnected depending on the previous result.

If previous xfer socket was disconnected, the 'ret' variable wasn't cleared
when the current socket is inactive. The outcome of this is that the
innocent transfer could be discarded.

Thanks to Jonathan Hoppe from Netriplex for discovering this.

src/knot/server/xfr-handler.c

@@ -1114,6 +1114,10 @@ int xfr_worker(dthread_t *thread)
 					ret = xfr_async_finish(&set, i);
 				else
 					ret = xfr_process_event(w, rq);
+			} else {
+				/* Inactive connection. */
+				++i;
+				continue;
 			}
 
 			/* Check task state. */