Fix crash on NodeTreeBase::parse_date_id()

[ma8ma]

スレッド読み込み中に異常終了したためstd::string_viewの部分文字列を取得する処理を修正します。

gcc12.2 gdbのバックトレース

#10 0x0000555555c32acf in std::__sv_check(unsigned long, unsigned long, char const*)
    (__size=243, __pos=244, __s=0x55555598ca42 "basic_string_view::substr") at /usr/include/c++/12/string_view:67
#11 0x0000555555c32fa2 in std::basic_string_view<char, std::char_traits<char> >::substr(unsigned long, unsigned long) const
    (this=0x7fffe97fe210, __pos=244, __n=18446744073709551615) at /usr/include/c++/12/string_view:311
#12 0x0000555555c7d072 in DBTREE::NodeTreeBase::parse_date_id(DBTREE::NODE*, std::basic_string_view<char, std::char_traits<char> >)
(gdb) frame 12
#12 0x0000555555c7d072 in DBTREE::NodeTreeBase::parse_date_id (this=0x5555577c19f0, header=0x555558e6bbe8,
2019                str = str.substr( start_block + lng_block );
(gdb) p str.size()
$3 = 243
(gdb) p start_block + lng_block
$4 = 244