dont predict missing timestamps if we lack the required information to do so

Originally committed as revision 3898 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -1115,7 +1115,7 @@ static int output_packet(AVInputStream *ist, int ist_index,
     } else if (pkt->dts != AV_NOPTS_VALUE) { //FIXME seems redundant, as libavformat does this too
         ist->next_pts = ist->pts = pkt->dts;
     } else {
-        assert(ist->pts == ist->next_pts);
+//        assert(ist->pts == ist->next_pts);
     }
     
     if (pkt == NULL) {

libavformat/utils.c

@@ -745,7 +745,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
         st->last_IP_pts= pkt->pts;
         /* cannot compute PTS if not present (we can compute it only
            by knowing the futur */
-    } else {
+    } else if(pkt->pts != AV_NOPTS_VALUE || pkt->dts != AV_NOPTS_VALUE || pkt->duration){
         if(pkt->pts != AV_NOPTS_VALUE && pkt->duration){
             int64_t old_diff= ABS(st->cur_dts - pkt->duration - pkt->pts);
             int64_t new_diff= ABS(st->cur_dts - pkt->pts);