MdeModulePkg/UdfDxe: Add more check when getting PD from LongAd

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1254

This commit will add an additional check within function GetPdFromLongAd()
when getting a Partition Descriptor from given a Long Allocation
Descriptor.

According to UDF 2.60 Spec, Section 2.2.13:

> The partition reference numbers used are determined by the order of the
> Partition Maps in the LVD.

(Also the picture comes before the above contents)

And a more detailed explanation of the partition reference numbers is at
https://sites.google.com/site/udfintro/ (seems not a formal documentation
though), Section 5.3.6.

Based on the above findings, the 'PartitionReferenceNumber' field in a
Long Allocation Descriptor is used as an index to access the Partition
Maps data within a Logical Volume Descriptor.

Hence, the new check focuses on the validity of this
'PartitionReferenceNumber' field in a Long Allocation Descriptor. Since
the current implementation of UdfDxe driver supports only one partition on
a Logical Volume, so the value of 'PartitionReferenceNumber' should be 0.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Paulo Alcantara <palcantara@suse.de>
Acked-by: Star Zeng <star.zeng@intel.com>

MdeModulePkg/Universal/Disk/UdfDxe/FileSystemOperations.c

@@ -242,11 +242,16 @@ GetPdFromLongAd (
     //
     // NOTE: Only one Type 1 (Physical) Partition is supported. It has been
     // checked already in Partition driver for existence of a single Type 1
-    // Partition map, so we don't have to double check here.
+    // Partition map. Hence, the 'PartitionReferenceNumber' field (the index
+    // used to access Partition Maps data within the Logical Volume Descriptor)
+    // in the Long Allocation Descriptor should be 0 to indicate there is only
+    // one partition.
     //
-    // Partition reference number can also be retrieved from
-    // LongAd->ExtentLocation.PartitionReferenceNumber, however the spec says
-    // it may be 0, so let's not rely on it.
+    if (LongAd->ExtentLocation.PartitionReferenceNumber != 0) {
+      return NULL;
+    }
+    //
+    // Since only one partition, get the first one directly.
     //
     PartitionNum = *(UINT16 *)((UINTN)&LogicalVolDesc->PartitionMaps[4]);
     break;