ffmpeg: reset input_ts_offset between files.

Signed-off-by: Alex Converse <alex.converse@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -3242,6 +3242,7 @@ static int opt_input_file(const char *opt, const char *filename)
     audio_channels    = 0;
     audio_sample_fmt  = AV_SAMPLE_FMT_NONE;
     av_dict_free(&ts_scale);
+    input_ts_offset = 0;
 
     for (i = 0; i < orig_nb_streams; i++)
         av_dict_free(&opts[i]);