lavf: Only initialize s->offset once when using avoid_negative_ts make_zero

When given a stream starting at dts=0, it would previously consider
s->offset as uninitialized and set an offset when the second packet
was written, ending up writing two packets with dts=0. By initializing
this field to AV_NOPTS_VALUE, we make sure that we only initialize it
once, on the first packet.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/mux.c

@@ -329,12 +329,12 @@ static int write_packet(AVFormatContext *s, AVPacket *pkt)
         AVRational time_base = s->streams[pkt->stream_index]->time_base;
         int64_t offset = 0;
 
-        if (!s->offset && pkt->dts != AV_NOPTS_VALUE &&
+        if (s->offset == AV_NOPTS_VALUE && pkt->dts != AV_NOPTS_VALUE &&
             (pkt->dts < 0 || s->avoid_negative_ts == AVFMT_AVOID_NEG_TS_MAKE_ZERO)) {
             s->offset = -pkt->dts;
             s->offset_timebase = time_base;
         }
-        if (s->offset)
+        if (s->offset != AV_NOPTS_VALUE)
             offset = av_rescale_q(s->offset, s->offset_timebase, time_base);
 
         if (pkt->dts != AV_NOPTS_VALUE)

libavformat/options.c

@@ -101,6 +101,7 @@ AVFormatContext *avformat_alloc_context(void)
     ic = av_malloc(sizeof(AVFormatContext));
     if (!ic) return ic;
     avformat_get_context_defaults(ic);
+    ic->offset = AV_NOPTS_VALUE;
 
     ic->internal = av_mallocz(sizeof(*ic->internal));
     if (!ic->internal) {