Crash if glyph is missing.

[paulhoux]

I tried to render Japanese text using Arial, but the application crashed here and here.

It could not find the glyph index, so iter becomes end. To me, it looks like missing glyphs are not automatically added to the texture yet.

[chaoticbob]

I added a skip for now: bdf4643.

I'll add some logic to add missing glyphs soon.