src: fix console debug output on Windows

The FWrite function on Windows assumed that MultiByteToWideChar automatically null-terminates the resulting string, but it will only do so if the size of the source was passed as -1 or null character was explicitly counted in the size. The FWrite uses std::string and its .size() method which doesn't count the null character even though the .data() method adds it to the resulting string.

https://docs.microsoft.com/en-us/windows/win32/api/stringapiset/nf-stringapiset-multibytetowidechar#remarks

MultiByteToWideChar does not null-terminate an output string if the input string length is explicitly specified without a terminating null character. To null-terminate an output string for this function, the application should pass in -1 or explicitly count the terminating null character for the input string.

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Discovered while debugging http2 tests on windows, all native logs were in one line. Hopefully I didn't miss another thing in the MultiByteToWideChar doc, it's quite confusing.

/cc @addaleax

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines