usb-storage: don't call usb_packet_complete twice

usb_msd_copy_data() may cause a recursive call to
usb_msd_command_complete() which in turn may complete
the packet, setting s->packet to NULL in case it does.
Recheck s->packet before calling usb_packet_complete()
to fix the double call.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb-msd.c

@@ -253,7 +253,7 @@ static void usb_msd_command_complete(SCSIBus *bus, int reason, uint32_t tag,
     s->scsi_buf = s->scsi_dev->info->get_buf(s->scsi_dev, tag);
     if (p) {
         usb_msd_copy_data(s);
-        if (s->usb_len == 0) {
+        if (s->packet && s->usb_len == 0) {
             /* Set s->packet to NULL before calling usb_packet_complete
                because another request may be issued before
                usb_packet_complete returns.  */