C#: Make sure that the file exists before attempting to get version info.

Author: michaelnebel

csharp/extractor/Semmle.Extraction.CSharp/Extractor/CompilerVersion.cs

@@ -76,7 +76,7 @@ public CompilerVersion(Options options)
 
                 // If csc is specified as compiler name, then attempt to read the version information from csc.dll
                 var compilerBinaryName = Path.GetFileName(SpecifiedCompiler) == "csc" ? $"{SpecifiedCompiler}.dll" : SpecifiedCompiler;
-                var versionInfo = FileVersionInfo.GetVersionInfo(compilerBinaryName);
+                var versionInfo = FileVersionInfo.GetVersionInfo(File.Exists(compilerBinaryName) ? compilerBinaryName : SpecifiedCompiler);
                 if (!knownCompilerNames.TryGetValue(versionInfo.OriginalFilename ?? string.Empty, out var vendor))
                 {
                     SkipExtractionBecause("the compiler name is not recognised");