avdovi/dovi_rpudec: handle prev_vdr_rpu_id failures

According to the spec, missing previous VDR RPU IDs do not constitute an
error, but we should instead fallback first to VDR RPU with ID 0, and
failing that, synthesize "neutral" metadata.

That's nontrivial though as the resulting metadata will be dependent on
other properties of the RPU, and this case is not hit in practice so
I'll defer it to a rainy day.

libavcodec/dovi_rpudec.c

@@ -456,7 +456,12 @@ int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, size_t rpu_size,
     if (use_prev_vdr_rpu) {
         int prev_vdr_rpu_id = get_ue_golomb_31(gb);
         VALIDATE(prev_vdr_rpu_id, 0, DOVI_MAX_DM_ID);
+        if (!s->vdr[prev_vdr_rpu_id])
+            prev_vdr_rpu_id = 0;
         if (!s->vdr[prev_vdr_rpu_id]) {
+            /* FIXME: Technically, the spec says that in this case we should
+             * synthesize "neutral" vdr metadata, but easier to just error
+             * out as this corner case is not hit in practice */
             av_log(s->logctx, AV_LOG_ERROR, "Unknown previous RPU ID: %u\n",
                    prev_vdr_rpu_id);
             goto fail;