MdeModulePkg AtaAtapiPassThru: Skip the potential NULL pointer access

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3732
Recent change c9742578 exposes this potential issue.

Signed-off-by: Liming Gao <gaoliming@byosoft.com.cn>
Acked-by: Rebecca Cran <rebecca@nuviainc.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>