Fix NRE in AssemblyNameReference

ICSharpCode.Decompiler/Metadata/AssemblyReferences.cs

@@ -79,7 +79,7 @@ namespace ICSharpCode.Decompiler.Metadata
 				builder.Append(Name);
 				builder.Append(sep);
 				builder.Append("Version=");
-				builder.Append(Version.ToString(fieldCount: 4));
+				builder.Append((Version ?? UniversalAssemblyResolver.ZeroVersion).ToString(fieldCount: 4));
 				builder.Append(sep);
 				builder.Append("Culture=");
 				builder.Append(string.IsNullOrEmpty(Culture) ? "neutral" : Culture);

ICSharpCode.Decompiler/Metadata/UniversalAssemblyResolver.cs

@@ -248,6 +248,8 @@ namespace ICSharpCode.Decompiler.Metadata
 			return version.Major == 0 && version.Minor == 0 && version.Build == 0 && version.Revision == 0;
 		}
 
+		internal static Version ZeroVersion = new Version(0,0,0,0);
+
 		string GetCorlib(IAssemblyReference reference)
 		{
 			var version = reference.Version;