Remove a magic number (#3888)

include/nlohmann/detail/output/serializer.hpp

@@ -927,7 +927,7 @@ class serializer
                 : (0xFFu >> type) & (byte);
 
         const std::size_t index = 256u + static_cast<size_t>(state) * 16u + static_cast<size_t>(type);
-        JSON_ASSERT(index < 400);
+        JSON_ASSERT(index < utf8d.size());
         state = utf8d[index];
         return state;
     }