Fix overwriting buffer

[terence1990]

@diegomura

.slice() is an in-place js function, it overwrites the buffer.

To test decode same buffer twice:

jay.decode(file);
jay.decode(file);

Current output:

{
  entries: {
    orientation: 6,
    xResolution: [ 72 ],
    yResolution: [ 72 ],
    resolutionUnit: 2,
    ycbCrPositioning: 1,
    exifIFDPointer: 102
  }
}
{
  entries: {
    exifVersion: null,
    componentsConfiguration: null,
    flashpixVersion: null,
    colorSpace: 65535,
    pixelXDimension: 801,
    pixelYDimension: 601
  }
}
{ entries: { undefined: 102 } }

Output after fix:

{
  entries: {
    orientation: 6,
    xResolution: [ 72 ],
    yResolution: [ 72 ],
    resolutionUnit: 2,
    ycbCrPositioning: 1,
    exifIFDPointer: 102
  }
}
{
  entries: {
    exifVersion: null,
    componentsConfiguration: null,
    flashpixVersion: null,
    colorSpace: 65535,
    pixelXDimension: 801,
    pixelYDimension: 601
  }
}
{
  entries: {
    orientation: 6,
    xResolution: [ 72 ],
    yResolution: [ 72 ],
    resolutionUnit: 2,
    ycbCrPositioning: 1,
    exifIFDPointer: 102
  }
}
{
  entries: {
    exifVersion: null,
    componentsConfiguration: null,
    flashpixVersion: null,
    colorSpace: 65535,
    pixelXDimension: 801,
    pixelYDimension: 601
  }
}