ShellPkg: Avoid buffer out-of-bound access
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 <9fa3a6894a36bca79e6a72ad65af5a5702efecd1@intel.com>
Reviewed-by: Steven Shi <4f7a0884debfd9139745b184e69c58dae2b3e6ce@intel.com>

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