MdeModulePkg/UdfDxe: Resolve potential NULL pointer dereference

MdeModulePkg/UdfDxe: Resolve potential NULL pointer dereference

Within function GetAllocationDescriptorLsn():

The call to GetPdFromLongAd() may return NULL and it will be later
dereferenced in GetShortAdLsn().

This commit adds ASSERT to resolve the potential NULL pointer
dereference.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Paulo Alcantara <pcacjr@zytor.com>