Pango: Really fix the check for zero-width chars.

Pango: Really fix the check for zero-width chars.

The value of 0x0FFFFFFF is assigned to a PangoGlyph if the current
character has zero width, so skip the glyph lookup stage for these
characters.

Note that 0xFFFFFFFF is used for invalid characters, so the error
messages for these cases are accurate.