Avoid duplicate thread recycling.

    At the end of the test, the traffic thread has been reclaimed.
If there is an exception in the control connection, it will cause
the thread to be reclaimed repeatedly.
Use sp->done to avoid repeated thread recycling.

src/iperf_client_api.c

@@ -807,6 +807,9 @@ iperf_run_client(struct iperf_test * test)
     /* Cancel all outstanding threads */
     i_errno_save = i_errno;
     SLIST_FOREACH(sp, &test->streams, streams) {
+        if (sp->done) {
+            continue;
+        }
         sp->done = 1;
         int rc;
         rc = pthread_cancel(sp->thr);