Pak file fixes

- FPakVFS::LoadPakIndex now handling "unused" pak values
- FPakVFS::DecryptDataBlock crashes with "FString[0/0] error when AES was not provided, now displaying more informative error

Unreal/FileSystem/UnArchivePak.cpp

@@ -684,6 +684,7 @@ void FPakVFS::DecryptDataBlock(byte* Data, int DataSize)
 	guard(FPakVFS::DecryptDataBlock);
 
 	const FString& Key = GetPakEncryptionKey();
+	if (Key.Len() == 0) appError("Empty AES key");
 	appDecryptAES(Data, DataSize, &Key[0], Key.Len());
 
 	unguard;
@@ -967,19 +968,25 @@ bool FPakVFS::LoadPakIndex(FArchive* reader, const FPakInfo& info, FString& erro
 			FolderIndex = RegisterGameFolder(*DirectoryPath);
 		}
 
-		for (int DirectoryFileIndex = 0; DirectoryFileIndex < NumFilesInDirectory; DirectoryFileIndex++)
+		for (int DirectoryFileIndex = 0; DirectoryFileIndex < NumFilesInDirectory; DirectoryFileIndex++, FileIndex++)
 		{
 			guard(File);
 			// Read FPakDirectory entry Key
 			FStaticString<MAX_PACKAGE_PATH> DirectoryFileName;
 			InfoReader << DirectoryFileName;
+
 			// Read FPakDirectory entry Value
+			// PakEntryLocation is positive (offset in 'EncodedPakEntries') or negative (index in 'Files').
 			int32 PakEntryLocation;
 			InfoReader << PakEntryLocation;
+			if (PakEntryLocation == 0x7fffffff || PakEntryLocation == 0x80000000)
+			{
+				// 0x7fffffff and 0x80000000 are special values, "Invalid" or "Unused"
+				continue;
+			}
 
 			FPakEntry& E = FileInfos[FileIndex];
 
-			// PakEntryLocation is positive (offset in 'EncodedPakEntries') or negative (index in 'Files')
 			// References in UE4:
 			// FPakFile::DecodePakEntry <- FPakFile::GetPakEntry (decode or pick from 'Files') <- FPakFile::Find (name to index/location)
 			if (PakEntryLocation < 0)
@@ -1012,7 +1019,6 @@ bool FPakVFS::LoadPakIndex(FArchive* reader, const FPakInfo& info, FString& erro
 			reg.IndexInArchive = FileIndex;
 			E.FileInfo = RegisterFile(reg);
 
-			FileIndex++;
 			unguard;
 		}
 		unguard;