display/qxl-render: fix race condition in qxl_cursor (CVE-2021-4207)

Avoid fetching 'width' and 'height' a second time to prevent possible
race condition. Refer to security advisory
https://starlabs.sg/advisories/22-4207/

 for more information.

Fixes: CVE-2021-4207
Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220407081106.343235-1-mcascell@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/display/qxl-render.c

@@ -266,7 +266,7 @@ static QEMUCursor *qxl_cursor(PCIQXLDevice *qxl, QXLCursor *cursor,
         }
         break;
     case SPICE_CURSOR_TYPE_ALPHA:
-        size = sizeof(uint32_t) * cursor->header.width * cursor->header.height;
+        size = sizeof(uint32_t) * c->width * c->height;
         qxl_unpack_chunks(c->data, size, qxl, &cursor->chunk, group_id);
         if (qxl->debug > 2) {
             cursor_print_ascii_art(c, "qxl/alpha");