`<xloctime>`: `time_get::get` can still assert 'istreambuf_iterator is not dereferenceable' when the format is longer than the stream

[StephanTLavavej]

Issue #1071 was thought to be resolved by #1168 in VS 2019 16.8 with tests added by #1326, but this wasn't completely fixed. With today's main:

D:\GitHub\STL\out\build\x64>type meow.cpp

#include <ctime>
#include <iostream>
#include <iterator>
#include <locale>
#include <sstream>
#include <string>
using namespace std;

int main() {
    const locale loc{locale::classic()};
    const auto& tmget{use_facet<time_get<char>>(loc)};
    ios_base::iostate err{ios_base::goodbit};
    tm when{};
    const string fmt{"%X"};
    istringstream iss{"3:04"}; // "3:04:05" would succeed
    istreambuf_iterator<char> first{iss};
    const istreambuf_iterator<char> last{};

    tmget.get(first, last, iss, err, &when, fmt.data(), fmt.data() + fmt.size());

    if (err == ios_base::goodbit) {
        cout << "when.tm_hour: " << when.tm_hour << "\n";
        cout << "when.tm_min:  " << when.tm_min << "\n";
        cout << "when.tm_sec:  " << when.tm_sec << "\n";
    } else {
        cout << "time_get::get() failed.\n";
    }
}

D:\GitHub\STL\out\build\x64>cl /EHsc /nologo /W4 /MTd meow.cpp
meow.cpp

D:\GitHub\STL\out\build\x64>meow
---------------------------
Microsoft Visual C++ Runtime Library
---------------------------
Debug Assertion Failed!

Program: D:\GitHub\STL\out\build\x64\meow.exe
File: D:\GitHub\STL\out\build\x64\out\inc\iterator
Line: 399

Expression: istreambuf_iterator is not dereferenceable

For information on how your program can cause an assertion
failure, see the Visual C++ documentation on asserts.

(Press Retry to debug the application)

---------------------------
Abort   Retry   Ignore   
---------------------------

"%X" "parses the locale's standard time representation" (cppreference), which is why "3:04" is insufficient but "3:04:05" would succeed.

Compiler Explorer shows that Clang/libc++ and GCC/libstdc++ behave correctly here, printing "time_get::get() failed.": https://godbolt.org/z/anv7TTz4G

Originally reported as DevCom-362423 and Microsoft-internal VSO-713785 / AB#713785 .