MdeModulePkg/PeiMain: PeiAllocatePool: output NULL if HOB creation fails

MdeModulePkg/PeiMain: PeiAllocatePool: output NULL if HOB creation fails

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1901

The original logic is ASSERT if fail to create HOB. But
that doesn't make sense for release version. So it is required
to set the Buffer to null to indicate the failure.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Turner <Michael.Turner@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Acked-by: Hao A Wu <hao.a.wu@intel.com>