av1_metadata: Error out if fragment is empty

If the fragment is empty after parsing (i.e. it contains no OBUs), then
the check for the type of the fragment's first OBU is nonsensical; so
error out in this case just as h264_metadata and hevc_metadata do.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/av1_metadata_bsf.c

@@ -133,6 +133,12 @@ static int av1_metadata_filter(AVBSFContext *bsf, AVPacket *pkt)
         goto fail;
     }
 
+    if (frag->nb_units == 0) {
+        av_log(bsf, AV_LOG_ERROR, "No OBU in packet.\n");
+        err = AVERROR_INVALIDDATA;
+        goto fail;
+    }
+
     for (i = 0; i < frag->nb_units; i++) {
         if (frag->units[i].type == AV1_OBU_SEQUENCE_HEADER) {
             obu = frag->units[i].content;