ShellPkg/for: Fix potential null pointer deference
ShellPkg/for: Fix potential null pointer deference

When "FOR %a %a IN A B C" is executed,
CurrentScriptFile->CurrentCommand->Data is NULL.
But the code assumes it's not NULL and tries to
deference it.

The patch fixes this issue.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <9fa3a6894a36bca79e6a72ad65af5a5702efecd1@intel.com>
Reviewed-by: Jaben Carsey <2d4ee76b36e91bee3a58378b7538904eb027a616@intel.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2