🚨 fixed some warnings

include/nlohmann/detail/output/serializer.hpp

@@ -620,7 +620,7 @@ class serializer
         if (is_negative)
         {
             *buffer_ptr = '-';
-            abs_value = static_cast<number_unsigned_t>(-1 - x) + 1;
+            abs_value = static_cast<number_unsigned_t>(std::abs(static_cast<std::intmax_t>(x)));
 
             // account one more byte for the minus sign
             n_chars = 1 + count_digits(abs_value);