Fix ArgumentOutOfRangeException on unexpected file in GAC

ICSharpCode.Decompiler/Metadata/UniversalAssemblyResolver.cs

@@ -723,7 +723,12 @@ namespace ICSharpCode.Decompiler.Metadata
 						continue;
 					foreach (var item in new DirectoryInfo(rootPath).EnumerateFiles("*.dll", SearchOption.AllDirectories))
 					{
-						string[]? name = Path.GetDirectoryName(item.FullName)?.Substring(rootPath.Length + 1).Split(new[] { "\\" }, StringSplitOptions.RemoveEmptyEntries);
+						// The root of the GAC should only contain folders, but make sure we handle the case where it does NOT in case
+						// someone has a non-standard layout (e.g. due to a broken installer).
+						string? assemblyParentPath = Path.GetDirectoryName(item.FullName);
+						if (assemblyParentPath?.Length > rootPath.Length)
+						{
+							string[]? name = assemblyParentPath.Substring(rootPath.Length + 1).Split(new[] { "\\" }, StringSplitOptions.RemoveEmptyEntries);
 							if (name?.Length != 2)
 								continue;
 							var match = Regex.Match(name[1], $"(v4.0_)?(?<version>[^_]+)_(?<culture>[^_]+)?_(?<publicKey>[^_]+)");
@@ -737,6 +742,7 @@ namespace ICSharpCode.Decompiler.Metadata
 					}
 				}
 			}
+		}
 
 		#endregion
 	}