FlatModelProxy: Fix bug in index computation.
FlatModelProxy: Fix bug in index computation.

freedesktop-unofficial-mirror/telepathy__telepathy-qt4-yell,mck182/telepathy-qt4-yell,mck182/telepathy-qt4-yell,freedesktop-unofficial-mirror/telepathy__telepathy-qt4-yell,freedesktop-unofficial-mirror/telepathy__telepathy-qt4-yell,mck182/telepathy-qt4-yell,freedesktop-unofficial-mirror/telepathy__telepathy-qt4-yell,mck182/telepathy-qt4-yell