avcodec/utils: fix AVPacket lifetime in seek_frame_generic

Fixes ticket #5117

libavformat/utils.c

@@ -2142,15 +2142,18 @@ static int seek_frame_generic(AVFormatContext *s, int stream_index,
             } while (read_status == AVERROR(EAGAIN));
             if (read_status < 0)
                 break;
-            av_packet_unref(&pkt);
             if (stream_index == pkt.stream_index && pkt.dts > timestamp) {
-                if (pkt.flags & AV_PKT_FLAG_KEY)
+                if (pkt.flags & AV_PKT_FLAG_KEY) {
+                    av_packet_unref(&pkt);
                     break;
+                }
                 if (nonkey++ > 1000 && st->codec->codec_id != AV_CODEC_ID_CDGRAPHICS) {
                     av_log(s, AV_LOG_ERROR,"seek_frame_generic failed as this stream seems to contain no keyframes after the target timestamp, %d non keyframes found\n", nonkey);
+                    av_packet_unref(&pkt);
                     break;
                 }
             }
+            av_packet_unref(&pkt);
         }
         index = av_index_search_timestamp(st, timestamp, flags);
     }