Fix a global buffer overflow in GCPDF_CIDFont::_CharCodeFromUnicode

There is not a code page (CP) used for converting unicode to mutli-bytes
if the coding scheme is CID coding. Only return 0 if CID can't be retrieved.
The difference on Windows and other platforms should be the function used
for converting rather than others.

BUG=466790
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1074653002