Add exception handling in text-controller

There is a case where mGlyphPositions is cleared in Controller::Relayouter::Relayout.
At this time, if try to partially erase mGlyphPositions in ClearGlyphModelData, Assert occurs.
Because the count of mGlyphPositions is already 0.

This patch prevents this issue though a count check.

Change-Id: I00c919c75acd1d389877c55acfcd75878b28bc91
Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>