Fix outputting extreme integer values in edge cases

For some gcc version (Ubuntu 5.5.0-12ubuntu1~16.04) the existing code
crashes when the minimum value of int64_t is outputted.

Rewrite the code to be less complicated to avoid it. This partially
reverts what was done in 546e2cb (:rotating_light: fixed some warnings,
2019-03-13).

Author: t-b

include/nlohmann/detail/output/serializer.hpp

@@ -630,7 +630,7 @@ class serializer
         if (is_negative)
         {
             *buffer_ptr = '-';
-            abs_value = static_cast<number_unsigned_t>(std::abs(static_cast<std::intmax_t>(x)));
+            abs_value = static_cast<number_unsigned_t>(-x);
 
             // account one more byte for the minus sign
             n_chars = 1 + count_digits(abs_value);

single_include/nlohmann/json.hpp

@@ -14264,7 +14264,7 @@ class serializer
         if (is_negative)
         {
             *buffer_ptr = '-';
-            abs_value = static_cast<number_unsigned_t>(std::abs(static_cast<std::intmax_t>(x)));
+            abs_value = static_cast<number_unsigned_t>(-x);
 
             // account one more byte for the minus sign
             n_chars = 1 + count_digits(abs_value);