ShellPkg/UefiShellLib: Check correct variable for NULL

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Westfahl <jeff.westfahl@ni.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
(cherry picked from commit 5220897839dd583e9d1f749fb4d8196ff86ccc8b)
1 file changed