shaping to 0-length glyph strings

[Havoc Pennington <hp redhat com>]

Hi,

Shape implementations are supposed to return a glyph string with at
least one glyph in it; I just added an assert to crash immediately if
this doesn't happen (previously it would cause crashes occasionally in
weird places).

Anyhow, known way to make it fail is to enter the character 0x1160
(holding shift-control and typing 1160 works to enter the char now).
This causes the Hangul shaper to return a zero-length glyph string on
my machine. This is VFILL in hangul.c. I don't know the right way to
fix it, if anyone does, please investigate.

Havoc