migration/multifd: fix nullptr access in multifd_send_terminate_threads

If the multifd_send_threads is not created when migration is failed,
multifd_save_cleanup would be called twice. In this senario, the
multifd_send_state is accessed after it has been released, the result
is that the source VM is crashing down.

Here is the coredump stack:
    Program received signal SIGSEGV, Segmentation fault.
    0x00005629333a78ef in multifd_send_terminate_threads (err=err@entry=0x0) at migration/ram.c:1012
    1012            MultiFDSendParams *p = &multifd_send_state->params[i];
    #0  0x00005629333a78ef in multifd_send_terminate_threads (err=err@entry=0x0) at migration/ram.c:1012
    #1  0x00005629333ab8a9 in multifd_save_cleanup () at migration/ram.c:1028
    #2  0x00005629333abaea in multifd_new_send_channel_async (task=0x562935450e70, opaque=<optimized out>) at migration/ram.c:1202
    #3  0x000056293373a562 in qio_task_complete (task=task@entry=0x562935450e70) at io/task.c:196
    #4  0x000056293373a6e0 in qio_task_thread_result (opaque=0x562935450e70) at io/task.c:111
    #5  0x00007f475d4d75a7 in g_idle_dispatch () from /usr/lib64/libglib-2.0.so.0
    #6  0x00007f475d4da9a9 in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
    #7  0x0000562933785b33 in glib_pollfds_poll () at util/main-loop.c:219
    #8  os_host_main_loop_wait (timeout=<optimized out>) at util/main-loop.c:242
    #9  main_loop_wait (nonblocking=nonblocking@entry=0) at util/main-loop.c:518
    #10 0x00005629334c5acf in main_loop () at vl.c:1810
    #11 0x000056293334d7bb in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4471

If the multifd_send_threads is not created when migration is failed.
In this senario, we don't call multifd_save_cleanup in multifd_new_send_channel_async.

Signed-off-by: Zhimin Feng <fengzhimin1@huawei.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/ram.c

@@ -1233,7 +1233,15 @@ static void multifd_new_send_channel_async(QIOTask *task, gpointer opaque)
     trace_multifd_new_send_channel_async(p->id);
     if (qio_task_propagate_error(task, &local_err)) {
         migrate_set_error(migrate_get_current(), local_err);
-        multifd_save_cleanup();
+        /* Error happen, we need to tell who pay attention to me */
+        qemu_sem_post(&multifd_send_state->channels_ready);
+        qemu_sem_post(&p->sem_sync);
+        /*
+         * Although multifd_send_thread is not created, but main migration
+         * thread neet to judge whether it is running, so we need to mark
+         * its status.
+         */
+        p->quit = true;
     } else {
         p->c = QIO_CHANNEL(sioc);
         qio_channel_set_delay(p->c, false);