nouveau: handle early initialization errors

handle very early errors in pipe_screen creation (failure of
nouveau_screen_init in nv50_screen_create)
Signed-off-by: Francisco Jerez <currojerez@riseup.net>

src/gallium/drivers/nouveau/nouveau_screen.c

@@ -258,6 +258,7 @@ nouveau_screen_fini(struct nouveau_screen *screen)
 {
 	struct pipe_winsys *ws = screen->base.winsys;
 	nouveau_channel_free(&screen->channel);
-	ws->destroy(ws);
+	if (ws)
+		ws->destroy(ws);
 }
 

src/gallium/winsys/nouveau/drm/nouveau_drm_winsys.c

@@ -19,7 +19,8 @@ nouveau_drm_destroy_winsys(struct pipe_winsys *s)
 {
 	struct nouveau_winsys *nv_winsys = nouveau_winsys(s);
 	struct nouveau_screen *nv_screen= nouveau_screen(nv_winsys->pscreen);
-	nouveau_device_close(&nv_screen->device);
+	if (nv_screen)
+		nouveau_device_close(&nv_screen->device);
 	FREE(nv_winsys);
 }