Lex: Avoid casting min_token_id to id_type in lexer ctor

[Kojoley]

When min_token_id is not a valid value for id_type and first_id is not provided lexer constructor is triggering UB. To fix this problem, constructor with ommited first_id initializes next_token_id directly, so if unique ids feature is not used there is no UB.

Found the problem in a lex test with UBSan.

boost/spirit/home/lex/lexer/lexer.hpp:382:27: runtime error: load of value 65536, which is not a valid value for type 'boost::spirit::lex::lexer<boost::spirit::lex::lexertl::actor_lexer<boost::spirit::lex::lexertl::token<__gnu_cxx::__normal_iterator<wchar_t *, std::__cxx11::basic_string<wchar_t> >, boost::mpl::vector<wchar_t, std::__cxx11::basic_string<wchar_t>, double, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, mpl_::bool_<true>, tokenids>, __gnu_cxx::__normal_iterator<wchar_t *, std::__cxx11::basic_string<wchar_t> >, boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<__gnu_cxx::__normal_iterator<wchar_t *, std::__cxx11::basic_string<wchar_t> >, boost::mpl::vector<wchar_t, std::__cxx11::basic_string<wchar_t>, double, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::    #0 0x44b481 in boost::spirit::lex::lexer<boost::spirit::lex::lexertl::actor_lexer<boost::spirit::lex::lexertl::token<__gnu_cxx::__normal_iterator<wchar_t*, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, boost::mpl::vector<wchar_t, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> >, double, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, mpl_::bool_<true>, tokenids>, __gnu_cxx::__normal_iterator<wchar_t*, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<__gnu_cxx::__normal_iterator<wchar_t*, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, boost::mpl::vector<wchar_t, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> >, double, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, mpl_::bool_<true>, tokenids>, boost::spirit::lex::lexertl::detail::data, __gnu_cxx::__normal_iterator<wchar_t*, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, mpl_::bool_<true>, mpl_::bool_<true> > > >::lexer(unsigned int, tokenids) ~/boost-root/./boost/spirit/home/lex/lexer/lexer.hpp:382:27
    #1 0x43a5cd in mega_tokens<boost::spirit::lex::lexertl::actor_lexer<boost::spirit::lex::lexertl::token<__gnu_cxx::__normal_iterator<wchar_t*, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, boost::mpl::vector<wchar_t, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> >, double, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, mpl_::bool_<true>, tokenids>, __gnu_cxx::__normal_iterator<wchar_t*, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<__gnu_cxx::__normal_iterator<wchar_t*, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, boost::mpl::vector<wchar_t, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> >, double, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, mpl_::bool_<true>, tokenids>, boost::spirit::lex::lexertl::detail::data, __gnu_cxx::__normal_iterator<wchar_t*, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, mpl_::bool_<true>, mpl_::bool_<true> > > >::mega_tokens() ~/boost-root/libs/spirit/test/lex/regression_wide.cpp:89:5
    #2 0x437bd5 in main ~/boost-root/libs/spirit/test/lex/regression_wide.cpp:122:29
    #3 0x7f078b0752e0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202e0)
    #4 0x40cff9 in _start (~/boost-root/bin.v2/libs/spirit/test/lex/lex_regression_wide.test/undefined/clang-linux-7/debug/visibility-hidden/lex_regression_wide+0x40cff9)

[hkaiser]

LGTM, thanks!