avformat/mux: Fix assertion failure due to max_interleave_delta and poor input

Fixes Ticket4051
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mux.c

@@ -574,7 +574,15 @@ static int write_packet(AVFormatContext *s, AVPacket *pkt)
         if (pkt->pts != AV_NOPTS_VALUE)
             pkt->pts += offset;
 
-        av_assert2(pkt->dts == AV_NOPTS_VALUE || pkt->dts >= 0);
+        av_assert2(pkt->dts == AV_NOPTS_VALUE || pkt->dts >= 0 || s->max_interleave_delta > 0);
+        if (pkt->dts != AV_NOPTS_VALUE && pkt->dts < 0) {
+            av_log(s, AV_LOG_WARNING,
+                   "Packets poorly interleaved, failed to avoid negative timestamp %s in stream %d\n"
+                   "try -max_interleave_delta 0 as a possible workaround\n",
+                   av_ts2str(pkt->dts),
+                   pkt->stream_index
+            );
+        }
     }
 
     did_split = av_packet_split_side_data(pkt);