IntelFsp2Pkg: Fixed potentially NULL pointer accessing

IntelFsp2Pkg: Fixed potentially NULL pointer accessing

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1280

When copying IDT table in SecMain, the pointer might be
NULL so added the check to fix it.

Test: Verified on internal platform and boots successfully.

Cc: Jiewen Yao <Jiewen.yao@intel.com>
Cc: Desimone Nathaniel L <nathaniel.l.desimone@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>