Parsing -+nan[(n-char-seq-opt)] and -+inf[inity] succeeds with FASTFLOAT_ALLOWS_LEADING_PLUS

[dalle]

The following strings succeeds to parse when FASTFLOAT_ALLOWS_LEADING_PLUS is defined:

"-+nan" // parsed as nan
"-+nan(abc)" // parsed as nan
"-+inf" // parsed as -inf
"-+infinity" // parsed as -inf

Mixing both - and + should result in an error.

To resolve this, replace:

  bool minusSign = false;
  if (*first ==
      UC('-')) { // assume first < last, so dereference without checks;
                 // C++17 20.19.3.(7.1) explicitly forbids '+' here
    minusSign = true;
    ++first;
  }
#ifdef FASTFLOAT_ALLOWS_LEADING_PLUS // disabled by default
  if (*first == UC('+')) {
    ++first;
  }
#endif

with:

  bool minusSign = (*first == UC('-'));
#ifdef FASTFLOAT_ALLOWS_LEADING_PLUS // disabled by default
  if ((*first == UC('-')) || (*first == UC('+'))) {
#else
  if (*first == UC('-')) {
#endif
    ++first;
  }

I'll create a PR with fix and additional tests.