ShellPkg: Use a local variable to cache the pointer.
CC: Qiu Shumin <shumin.qiu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Erik Bjorge <erik.c.bjorge@intel.com>
Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>