The decoder fails parsing zero-length boxes

[kapetan]

There is a bug where the decoder halts after encountering a zero-length box.

E.g. calling decode(function(box)) in the handler for the box event, never calls the provided callback if the box has a contentLen of zero. The ignore method is also affected by this.

This pull-request contains a failing test.

A quick solution for only fixing decode would be to add a zero-length check to the Decoder.prototype._buffer method.

Decoder.prototype._buffer = function (size, cb) {
  if (!size) return cb(new Buffer(0))      // This fixes the test. But ignore and stream are still affected.
  this._missing = size
  this._buf = new Buffer(size)
  this._cb = cb
}

I think similar checks could also be added to Decoder.prototype._stream and Decoder.prototype.ignore methods.