lavc/options: initialize pkt_timebase

It's default in option_table.h is 0, but without this fix it is represented as 0/0.
Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>

libavcodec/options.c

@@ -108,6 +108,7 @@ int avcodec_get_context_defaults3(AVCodecContext *s, const AVCodec *codec)
 
     s->time_base           = (AVRational){0,1};
     s->framerate           = (AVRational){ 0, 1 };
+    s->pkt_timebase        = (AVRational){ 0, 1 };
     s->get_buffer2         = avcodec_default_get_buffer2;
     s->get_format          = avcodec_default_get_format;
     s->execute             = avcodec_default_execute;