fix a crash with empty string
fix a crash with empty string

If it's named nLast, then it's supposed to point to the last character,
which, with an empty string, logically means it should point before
the start of the string.

Change-Id: I248fbe80c1c4b1c2c8d39f0bcf772f9b85415e4a

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core