vf_vapoursynth: fix obscure/impossible leak

Unknown frames were not freed properly. Although this doesn't really
happen anyway, because we're never going to feed audio frames to a video
filter chain. Since it's theoretically possible, and all other filters
handle this consistently, fix it anyway.

video/filter/vf_vapoursynth.c

@@ -379,6 +379,7 @@ static void vf_vapoursynth_process(struct mp_filter *f)
             pthread_cond_broadcast(&p->wakeup);
         } else if (frame.type != MP_FRAME_NONE) {
             MP_ERR(p, "discarding unknown frame type\n");
+            mp_frame_unref(&frame);
             goto done;
         }
     }