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>

MdeModulePkg/Universal/Disk/UdfDxe/FileSystemOperations.c

@@ -748,12 +748,17 @@ GetAllocationDescriptorLsn (
   IN VOID                            *Ad
   )
 {
+  UDF_PARTITION_DESCRIPTOR *PartitionDesc;
+
   if (RecordingFlags == LongAdsSequence) {
     return GetLongAdLsn (Volume, (UDF_LONG_ALLOCATION_DESCRIPTOR *)Ad);
   } else if (RecordingFlags == ShortAdsSequence) {
+    PartitionDesc = GetPdFromLongAd (Volume, ParentIcb);
+    ASSERT (PartitionDesc != NULL);
+
     return GetShortAdLsn (
       Volume,
-      GetPdFromLongAd (Volume, ParentIcb),
+      PartitionDesc,
       (UDF_SHORT_ALLOCATION_DESCRIPTOR *)Ad
       );
   }