Prevent a crash when looking up U+FDD0 and U+FDD1.

Qt internally uses U+FDD0 and U+FDD1 to mark the beginning and the end of frames.
They should be seen as non-printable characters, as trying to display them leads to a crash caused by a Qt "noBlockInString" assertion.

Thanks to <a href="http://xkcd.com/380/">http://xkcd.com/380/</a>; for pointing me to the problem. :) (Look at the tooltip.)