ShellPkg/UefiShellLib: Check correct variable for NULL
ShellPkg/UefiShellLib: Check correct variable for NULL

Cc: Ruiyu Ni <9fa3a6894a36bca79e6a72ad65af5a5702efecd1@intel.com>
Cc: Jaben Carsey <2d4ee76b36e91bee3a58378b7538904eb027a616@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Westfahl <928263be46c1cf5087ad2af69c2bb19bb5309d2a@ni.com>
Reviewed-by: Jaben Carsey <2d4ee76b36e91bee3a58378b7538904eb027a616@intel.com>

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