vout: keep the original picture if "early" SPU blending fails

This avoids loosing the picture context if there is one. If the
original picture has no context, it makes no difference whether the
original or the copy is kept since blending failed anyway.

src/video_output/video_output.c

@@ -975,8 +975,8 @@ static int ThreadDisplayRenderPicture(vout_thread_t *vout, bool is_forced)
         if (blent) {
             VideoFormatCopyCropAr(&blent->format, &filtered->format);
             picture_Copy(blent, filtered);
-            if (vout->p->spu_blend) {
-                picture_BlendSubpicture(blent, vout->p->spu_blend, subpic);
+            if (vout->p->spu_blend
+             && picture_BlendSubpicture(blent, vout->p->spu_blend, subpic)) {
                 picture_Release(todisplay);
                 todisplay = blent;
             } else