fdo#73095: fix invalid access in SwFntObj::DrawText()

aStr may be 1 larger than pKernArray.
Trivial fix by checking the largest index; not sure if it would be a
good idea to allocate pKernArray with the larger size in the first
place, but that would be a bigger change...
(regression from 02ce734450559c9353ca7f42b2519239220dd265)

Change-Id: Ia33feab001c34e85066b7596d8873f41588984e9