lavf: Fix the last PTS to be generated using -fflags +genpts

To reproduce the problem, using ffprobe:

./ffprobe -show_packets -print_format compact -fflags +genpts -i
fate_samples/mxf/C0023S01.mxf

You will notice that the last video frame does not have it's PTS being
set, even with using genpts.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -1351,14 +1351,30 @@ int av_read_frame(AVFormatContext *s, AVPacket *pkt)
 
             if (next_pkt->dts != AV_NOPTS_VALUE) {
                 int wrap_bits = s->streams[next_pkt->stream_index]->pts_wrap_bits;
+                // last dts seen for this stream. if any of packets following
+                // current one had no dts, we will set this to AV_NOPTS_VALUE.
+                int64_t last_dts = next_pkt->dts;
                 while (pktl && next_pkt->pts == AV_NOPTS_VALUE) {
                     if (pktl->pkt.stream_index == next_pkt->stream_index &&
-                        (av_compare_mod(next_pkt->dts, pktl->pkt.dts, 2LL << (wrap_bits - 1)) < 0) &&
-                         av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2LL << (wrap_bits - 1))) { //not b frame
-                        next_pkt->pts = pktl->pkt.dts;
+                        (av_compare_mod(next_pkt->dts, pktl->pkt.dts, 2LL << (wrap_bits - 1)) < 0)) {
+                        if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2LL << (wrap_bits - 1))) { //not b frame
+                            next_pkt->pts = pktl->pkt.dts;
+                        }
+                        if (last_dts != AV_NOPTS_VALUE) {
+                            // Once last dts was set to AV_NOPTS_VALUE, we don't change it.
+                            last_dts = pktl->pkt.dts;
+                        }
                     }
                     pktl = pktl->next;
                 }
+                if (eof && next_pkt->pts == AV_NOPTS_VALUE && last_dts != AV_NOPTS_VALUE) {
+                    // Fixing the last reference frame had none pts issue (For MXF etc).
+                    // We only do this when
+                    // 1. eof.
+                    // 2. we are not able to resolve a pts value for current packet.
+                    // 3. the packets for this stream at the end of the files had valid dts.
+                    next_pkt->pts = last_dts + next_pkt->duration;
+                }
                 pktl = s->packet_buffer;
             }