libfreerdp-core: fix reading TS_ICON_INFO

libfreerdp-core: fix reading TS_ICON_INFO

The spec says that CbColorTable field is present when Bpp is 1, 4, 8.
Actually, bpp == 2 is not supported by TS_ICON_INFO according to the
spec (though, DIB definitely supports 16-color images).

    MS-RDPERP 2.2.1.2.3 Icon Info (TS_ICON_INFO)

    CbColorTable (2 bytes):
        This field is ONLY present if the bits per pixel (Bpp)
        value is 1, 4, or 8.

Omitting 8-bit value breaks 256-color icons which are incorrectly
read with color and alpha data mixed up.