Avoid undefined std::abs(INT_MIN) in integer_parser

When parsing a negative integer into a signed type whose minimum value
equals INTMAX_MIN (e.g. int64_t on typical platforms), integer_parser
computes the limit as std::abs(static_cast<intmax_t>(min())). Negating
INTMAX_MIN cannot be represented in intmax_t, so the std::abs call is
undefined behaviour. UBSan flags this on every negative int64_t parse,
including a trivial one like "-1":

  runtime error: negation of -9223372036854775808 cannot be represented
  in type 'long'; cast to an unsigned type to negate this value to itself

Build the same unsigned magnitude (|min| == max + 1 for two's-complement)
directly in the unsigned arithmetic type instead. For unsigned T the
expression wraps to 0, which matches the previous std::abs(0) value, so
behaviour for unsigned types is preserved.

include/cxxopts.hpp

@@ -1057,7 +1057,11 @@ integer_parser(const std::string& text, T& value)
     US limit = 0;
     if (negative)
     {
-      limit = static_cast<US>(std::abs(static_cast<intmax_t>((std::numeric_limits<T>::min)())));
+      // |min| equals max + 1 for two's-complement signed types; computing it
+      // via std::abs(min) is undefined behaviour (e.g. abs(INT64_MIN)). Build
+      // the same value in unsigned arithmetic instead. For unsigned T this
+      // intentionally wraps to 0, matching the previous std::abs(0) result.
+      limit = static_cast<US>(static_cast<US>((std::numeric_limits<T>::max)()) + US{1});
     }
     else
     {