avcodec/hevc_ps: Only discard overread VPS if a previous is available

Fixes Ticket4621

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/hevc_ps.c

@@ -499,6 +499,7 @@ int ff_hevc_decode_nal_vps(HEVCContext *s)
     if (get_bits_left(gb) < 0) {
         av_log(s->avctx, AV_LOG_ERROR,
                "Overread VPS by %d bits\n", -get_bits_left(gb));
+        if (s->vps_list[vps_id])
             goto err;
     }