Fix #2544: do not transform to interpolated string, if the format string is not syntactically correct.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/CS6_StringInterpolation.cs

@@ -19,6 +19,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			Console.WriteLine($"\ta{args ?? args}");
 			Console.WriteLine($"\ta{args[0][0] == 'a'}");
 			Console.WriteLine($"\ta{$"a{args.Length}" == args[0]}");
+			Console.WriteLine($"\ta{args.Length}}}");
 		}
 
 		public static void ArrayExpansionSpecialCases(object[] args)
@@ -54,6 +55,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			Console.WriteLine(string.Format("{0:}", args.Length));
 			Console.WriteLine(string.Format("{0{a}0}", args.Length));
 			Console.WriteLine(string.Format("test: {0}", string.Join(",", args)));
+			Console.WriteLine(string.Format("test: {0}}", args.Length));
 #pragma warning restore
 		}
 

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -670,9 +670,14 @@ namespace ICSharpCode.Decompiler.CSharp
 							sb.Clear();
 							kind = TokenKind.String;
 						}
+						else if (Peek() == '}')
+						{
+							sb.Append("}}");
+							Next();
+						}
 						else
 						{
-							sb.Append((char)next);
+							yield return (TokenKind.Error, null);
 						}
 						break;
 					case ':':