ShellPkg: Avoid buffer out-of-bound access

PathSize is the number of bytes in PathForReturn buffer so
PathForReturn[PathSize - 1] incorrectly accesses the last
character in the buffer,
PathForReturn[PathSize / sizeof (CHAR16) - 1] should be used.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Steven Shi <steven.shi@intel.com>