MdeModulePkg/PciBus: Avoid hang when BUS pad resource is not in top

PciScanBus() assumes the GetResourcePadding() puts BUS descriptor
in the very beginning, if it's not, the Descriptors will be updated
to point to middle of the pool buffer, which can cause
FreePool(Descriptors) hang in DEBUG image.
No functionality impact to RELEASE image.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>