qemu-char: make tcp_chr_disconnect() reentrant-safe

During CHR_EVENT_CLOSED, the function could be reentered, make this
case safe.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

qemu-char.c

@@ -2769,6 +2769,10 @@ static void tcp_chr_disconnect(CharDriverState *chr)
 {
     TCPCharDriver *s = chr->opaque;
 
+    if (!s->connected) {
+        return;
+    }
+
     s->connected = 0;
     if (s->listen_ioc) {
         s->listen_tag = qio_channel_add_watch(