Revert "usb-storage: Drop useless null test in usb_msd_handle_data()"

This reverts commit a1cbfd55.

Test isn't useless.  scsi_req_enqueue() may finish the request (will
actually happen for requests which don't trigger any I/O such as
INQUIRY), then call usb_msd_command_complete() which in turn will
set s->req to NULL after unref'ing it.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/dev-storage.c

@@ -427,7 +427,7 @@ static void usb_msd_handle_data(USBDevice *dev, USBPacket *p)
             scsi_req_print(s->req);
 #endif
             scsi_req_enqueue(s->req);
-            if (s->req->cmd.xfer != SCSI_XFER_NONE) {
+            if (s->req && s->req->cmd.xfer != SCSI_XFER_NONE) {
                 scsi_req_continue(s->req);
             }
             break;