Fix get_glyph_shape panic consola \u{feff}

Panics with

let consolas = 
    stb_truetype::FontInfo::new(&include_bytes!("../consola.ttf")[..], 
0).ok_or("bad font")?;
let feff_id = consolas.find_glyph_index('\u{feff}' as u32);
let _ = consolas.get_glyph_shape(feff_id);

Fixes #19 (closed)