Set initial next_pts to unknown, this avoids the nonsense timestamp

discontinuity at the start. I hope this has no sideeffects, if it does
send a bugreport!
fixes issue137

Originally committed as revision 12134 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -1826,10 +1826,7 @@ static int av_encode(AVFormatContext **output_files,
         ist = ist_table[i];
         is = input_files[ist->file_index];
         ist->pts = 0;
-        ist->next_pts=0;
-        if(   input_files_ts_offset[ist->file_index] != -is->start_time
-           && !(is->start_time == AV_NOPTS_VALUE && input_files_ts_offset[ist->file_index]==0))
-            ist->next_pts= AV_NOPTS_VALUE;
+        ist->next_pts = AV_NOPTS_VALUE;
         ist->is_start = 1;
     }