Fix NullReferenceException when an interface method named 'Invoke' is called.

ICSharpCode.Decompiler/Ast/AstMethodBodyBuilder.cs

@@ -598,7 +598,7 @@ namespace ICSharpCode.Decompiler.Ast
 							};
 						}
 					}
-				} else if (cecilMethodDef.Name == "Invoke" && cecilMethodDef.DeclaringType.BaseType.FullName == "System.MulticastDelegate") {
+				} else if (cecilMethodDef.Name == "Invoke" && cecilMethodDef.DeclaringType.BaseType != null && cecilMethodDef.DeclaringType.BaseType.FullName == "System.MulticastDelegate") {
 					AdjustArgumentsForMethodCall(cecilMethod, methodArgs);
 					return target.Invoke(methodArgs);
 				}