hevc: check that VPS referenced from SPS exists

This matches how its done for SPS/PPS.
Fixes null pointer dereference.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/hevc_ps.c

@@ -633,6 +633,13 @@ int ff_hevc_decode_nal_sps(HEVCContext *s)
         goto err;
     }
 
+    if (!s->vps_list[sps->vps_id]) {
+        av_log(s->avctx, AV_LOG_ERROR, "VPS %d does not exist\n",
+               sps->vps_id);
+        ret = AVERROR_INVALIDDATA;
+        goto err;
+    }
+
     sps->max_sub_layers = get_bits(gb, 3) + 1;
     if (sps->max_sub_layers > MAX_SUB_LAYERS) {
         av_log(s->avctx, AV_LOG_ERROR, "sps_max_sub_layers out of range: %d\n",