ShellPkg: Initialize the local pointer to avoid potential suspicious dereference.

ShellPkg: Initialize the local pointer to avoid potential suspicious dereference.

1. Initialize the local pointer 'HandleBuffer'.
2. When 'LocateHandleBuffer' return error 'HandleBuffer' is expected unchanged, add code make sure this.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19229 6f19259b-4bc3-4df7-8a09-765794883524