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/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) {