fix fragment duration when no mehd is provided
fix fragment duration when no mehd is provided

jhiesey/mp4box.js,sjames1958gm/mp4box.js,b123400/mp4box.js,hanzeil/mp4box.js,emmanouil/mp4box.js-3d-old,cconcolato/mp4box.js,cconcolato/mp4box.js,gpac/mp4box.js,sjames1958gm/mp4box.js,emmanouil/mp4box.js-3d-old,b123400/mp4box.js,hanzeil/mp4box.js,jhiesey/mp4box.js,gpac/mp4box.js