vd_lavc: reset last_sample_aspect_ratio in uninit_avctx()

In init_vo(), if sh->aspect is 0 or last_sample_aspect_ratio is set,
sh->aspect is overwritten. With software decoding fallback behaviour,
this makes the aspect ratio from container ignored since
last_sample_aspect_ratio is already set in first try with hardware
decoding.

video/decode/vd_lavc.c

@@ -410,6 +410,7 @@ static void uninit_avctx(sh_video_t *sh)
 #if !HAVE_AVUTIL_REFCOUNTING
     mp_buffer_pool_free(&ctx->dr1_buffer_pool);
 #endif
+    ctx->last_sample_aspect_ratio = (AVRational){0, 0};
 }
 
 static void uninit(sh_video_t *sh)