[HB] Fix apply_lookup() loop
[HB] Fix apply_lookup() loop

Part of Bug 595539 - Regressions in rendering certain Thai sequences with
OpenType font

Distrotech/pango,Distrotech/pango,danny-ku/pango,kari-lentz/alpha-pango,kari-lentz/alpha-pango,OpenInkpot-archive/iplinux-pango1.0,kari-lentz/alpha-pango,OpenInkpot-archive/iplinux-pango1.0,OpenInkpot-archive/iplinux-pango1.0,Distrotech/pango,kari-lentz/alpha-pango,danny-ku/pango,danny-ku/pango