ShellPkg: Refine the logic about allocating memory for variable name and data.

ShellPkg: Refine the logic about allocating memory for variable name and data.

The run time service 'QueryVariableInfo' is not proper to be used to get the variable name size. This patch refine the logic about allocating memory for variable name and data.

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@17201 6f19259b-4bc3-4df7-8a09-765794883524