Fix calculation of max buffer length not accounting for negative signs

include/boost/charconv/detail/to_chars_integer_impl.hpp

@@ -89,8 +89,7 @@ BOOST_CHARCONV_CONSTEXPR to_chars_result to_chars_integer_impl(char* first, char
 
  char buffer[10] {};
  int converted_value_digits {};
- const std::ptrdiff_t user_buffer_size = last - first;
- BOOST_ATTRIBUTE_UNUSED bool is_negative = false;
+ bool is_negative = false;
 
  if (first > last)
  {
@@ -115,6 +114,8 @@ BOOST_CHARCONV_CONSTEXPR to_chars_result to_chars_integer_impl(char* first, char
  unsigned_value = static_cast<Unsigned_Integer>(value);
  }
 
+ const std::ptrdiff_t user_buffer_size = last - first - static_cast<std::ptrdiff_t>(is_negative);
+
  // If the type is less than 32 bits we can use this without change
  // If the type is greater than 32 bits we use a binary search tree to figure out how many digits
  // are present and then decompose the value into two (or more) std::uint32_t of known length so that we
@@ -314,8 +315,6 @@ BOOST_CHARCONV_CONSTEXPR to_chars_result to_chars_128integer_impl(char* first, c
 template <typename Integer, typename Unsigned_Integer>
 BOOST_CHARCONV_CONSTEXPR to_chars_result to_chars_integer_impl(char* first, char* last, Integer value, int base) noexcept
 {
- const std::ptrdiff_t output_length = last - first;
-
  if (!((first <= last) && (base >= 2 && base <= 36)))
  {
  return {last, std::errc::invalid_argument};
@@ -347,6 +346,8 @@ BOOST_CHARCONV_CONSTEXPR to_chars_result to_chars_integer_impl(char* first, char
  unsigned_value = static_cast<Unsigned_Integer>(value);
  }
 
+ const std::ptrdiff_t output_length = last - first;
+
  constexpr Unsigned_Integer zero = 48U; // Char for '0'
  constexpr auto buffer_size = sizeof(Unsigned_Integer) * CHAR_BIT;
  char buffer[buffer_size] {};