Assert for impossible case

tinyxml2.cpp

@@ -338,8 +338,9 @@ void XMLUtil::ConvertUTF32ToUTF8( unsigned long input, char* output, int* length
         case 1:
             --output;
             *output = (char)(input | FIRST_BYTE_MARK[*length]);
-        default:
             break;
+        default:
+            TIXMLASSERT( false );
     }
 }