vout: robustify "early" SPU blending

If the picture copy fails or blending is unavailable(1), discard only
the subpicture and keep the original picture.

(1) In practice, this never happens as the blender object is kept
all the time even without a "blend" module.

src/video_output/video_output.c

@@ -971,19 +971,19 @@ static int ThreadDisplayRenderPicture(vout_thread_t *vout, bool is_forced)
     bool is_direct = vout->p->decoder_pool == vout->p->display_pool;
     picture_t *todisplay = filtered;
     if (do_early_spu && subpic) {
-        todisplay = picture_pool_Get(vout->p->private_pool);
-        if (todisplay) {
-            VideoFormatCopyCropAr(&todisplay->format, &filtered->format);
-            picture_Copy(todisplay, filtered);
-            if (vout->p->spu_blend)
-                picture_BlendSubpicture(todisplay, vout->p->spu_blend, subpic);
+        picture_t *blent = picture_pool_Get(vout->p->private_pool);
+        if (blent) {
+            VideoFormatCopyCropAr(&blent->format, &filtered->format);
+            picture_Copy(blent, filtered);
+            if (vout->p->spu_blend) {
+                picture_BlendSubpicture(blent, vout->p->spu_blend, subpic);
+                picture_Release(todisplay);
+                todisplay = blent;
+            } else
+                picture_Release(blent);
         }
-        picture_Release(filtered);
         subpicture_Delete(subpic);
         subpic = NULL;
-
-        if (!todisplay)
-            return VLC_EGENERIC;
     }
 
     assert(vout_IsDisplayFiltered(vd) == !sys->display.use_dr);