ShellPkg: Verify memory and handle allocation failure.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: jaben carsey <jaben.carsey@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14448 6f19259b-4bc3-4df7-8a09-765794883524