Fix fetchGlyphsAndAdvances() in CoreText renderer

[ichizok]

This will fix #693

In fetchGlyphsAndAdvances() when count < offset, count is signed and offset is unsigned so count - offset is a huge value then count - offset > length is true and count becomes a wrong value (count = length - offset).
Thus CTRungetGlyphs() will fail.

I think it is the correct way to check if count > length - offset.