Fixed a bug in the glyph lookup.

[Issue]
  Button text is 'OO' not 'OK'.

[Cause]
  In this content, there's a character-code table in DefineFont.
  However, all the entries in the table are zero.
  So, every character is mapped to the first entry of the glyph list, 'O'.

[Fix]
  Changed to directly look up the glyph table when the char-code is zero.