Shellpkg/editor: Fix a bug that may modifies Line[-1]

Shellpkg/editor: Fix a bug that may modifies Line[-1]

The original code as below intend to set the character
before last column to CHAR_NULL.
  Line[(LastCol % (ARRAY_SIZE (Line) - 1)) - 1] = CHAR_NULL;

But when LastCol % (ARRAY_SIZE (Line) - 1)) equals to 0,
Line[-1] is modified.

We should change to code as below:
  Line[(LastCol - 1) % (ARRAY_SIZE (Line) - 1)] = CHAR_NULL;

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>