st/xorg: Prevent potential null pointer deference in xorg_xv.c.

src/gallium/state_trackers/xorg/xorg_xv.c

@@ -446,7 +446,7 @@ display_video(ScrnInfoPtr pScrn, struct xorg_xv_port_priv *pPriv, int id,
    int x, y, w, h;
    struct exa_pixmap_priv *dst = exaGetPixmapDriverPrivate(pPixmap);
 
-   if (!dst->tex) {
+   if (dst && !dst->tex) {
 	xorg_exa_set_shared_usage(pPixmap);
 	pScrn->pScreen->ModifyPixmapHeader(pPixmap, 0, 0, 0, 0, 0, NULL);
    }