C#: ZipSlip - Address review comments.

- Add backticks
 - Add extra test.

csharp/ql/src/semmle/code/csharp/security/dataflow/ZipSlip.qll

@@ -112,7 +112,7 @@ module ZipSlip {
   }
 
   /**
-   * A call to Substring.
+   * A call to `Substring`.
    *
    * This is considered a sanitizer because `Substring` may be used to extract a single component
    * of a path to avoid ZipSlip.

csharp/ql/test/query-tests/Security Features/CWE-022/ZipSlip/ZipSlip.cs

@@ -59,7 +59,7 @@ namespace ZipSlip
                     foreach (ZipArchiveEntry entry in archive.Entries)
                     {
                         // figure out where we are putting the file
-                        string destFilePath = Path.Combine(InstallDir, entry.FullName);
+                        String destFilePath = Path.Combine(InstallDir, entry.FullName);
 
                         Directory.CreateDirectory(Path.GetDirectoryName(destFilePath));
 
@@ -94,6 +94,15 @@ namespace ZipSlip
                                 Console.WriteLine(@"Writing ""{0}""", destFilePath);
                                 archiveFileStream.CopyTo(fs);
                             }
+
+                            // GOOD: Use substring to pick out single component
+                            string fileName = destFilePath.Substring(destFilePath.LastIndexOf("\\"));
+                            var fileInfo2 = new FileInfo(fileName);
+                            using (FileStream fs = fileInfo2.Open(FileMode.Create))
+                            {
+                                Console.WriteLine(@"Writing ""{0}""", destFilePath);
+                                archiveFileStream.CopyTo(fs);
+                            }
                         }
                     }
                 }