Fix #1615: Handle nil tokens gracefully.

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -124,9 +124,10 @@ namespace ICSharpCode.Decompiler.IL
 		EntityHandle ReadAndDecodeMetadataToken()
 		{
 			int token = reader.ReadInt32();
-			if (token < 0) {
+			if (token <= 0) {
 				// SRM uses negative tokens as "virtual tokens" and can get confused
 				// if we manually create them.
+				// Row-IDs < 1 are always invalid.
 				throw new BadImageFormatException("Invalid metadata token");
 			}
 			return MetadataTokens.EntityHandle(token);

ICSharpCode.Decompiler/IL/InstructionOutputExtensions.cs

@@ -69,8 +69,10 @@ namespace ICSharpCode.Decompiler.IL
 
 		public static void WriteTo(this EntityHandle entity, PEFile module, ITextOutput output, Metadata.GenericContext genericContext, ILNameSyntax syntax = ILNameSyntax.Signature)
 		{
-			if (entity.IsNil)
-				throw new ArgumentNullException(nameof(entity));
+			if (entity.IsNil) {
+				output.Write("<nil>");
+				return;
+			}
 			if (module == null)
 				throw new ArgumentNullException(nameof(module));
 			var metadata = module.Metadata;