MdeModulePkg VarCheckPcd: Make sure the bin at required alignment

According to UEFI/PI spec, the AllocateXXXPool() interfaces should
return eight-byte aligned buffer that satisfies the required
HEADER_ALIGNMENT which is 4.

This patch adds ASSERT to make sure the bin at required alignment,
that can help catch the issue earlier about incorrect aligned buffer
returned from AllocateXXXPool().

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>