Properly null-terminate output buffer in basic_istream::get[line]

[muellerj2]

Fixes #5070.

The libcxx tests istream.unformatted/get_pointer_size.pass.cpp and get_pointer_size_chart.pass.cpp pass now.

Since this issue is mostly about correct null-termination in the presence of exceptions, I added the tests to GH_001858_iostream_exception. They should cover the relevant cases:

• output buffer of size zero with and without exception on failbit
• sentry failure with and without exception on failbit
• exception during streambuf character extraction with and without badbit exception propagation
• encountering EOF with and without exception on eofbit

[muellerj2]

@microsoft-github-policy-service agree

[StephanTLavavej]

Thanks, this is fantastic - and incredible for a first PR! 😻 I've pushed several minor nitpicks, a product code simplification, and a test code improvement. I appreciate the extremely detailed test coverage.

We merge PRs simultaneously to our GitHub and MSVC-internal repos, mirrored in a semi-manual process and batched up to save time. Your PR will be part of the next batch, very likely this week.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks again, and congratulations on your first microsoft/STL commit! 🚀 😸 🥳

This change is expected to ship in VS 2022 17.13 Preview 3.