Build ESDS box missing data property

[hughfenghen]

Missing data causes the size to be calculated incorrectly, which ultimately prevents the file from being parsed.

Adding the code after https://github.com/gpac/mp4box.js/blob/master/src/parsing/esds.js#L2 seems to solve the problem this.data = esd_data

Releated: https://github.com/hughfenghen/WebAV/issues/10