ShellPkg: Fix LastError environment variable truncation

-Fixes LastError environment variable being truncated to 32 bits on 64 bit systems
-Fixes possible buffer overrun issue with LastError string value

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Robert Matthews <robert.matthews@hp.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14608 6f19259b-4bc3-4df7-8a09-765794883524