utf8_to_wstrbuf now handles invalid utf8 sequences

[captainurist]

Fixes #3241

[gabime]

Thanks @captainurist

[gabime]

@captainurist Seems that some of the new tests fail. Can you take a look?

[captainurist]

Yes, looking at it now.

[captainurist]

@gabime OK test failures are easy to fix, preparing a PR.

For the compilation errors, these make no sense to me:

C:\projects\spdlog\tests\test_misc.cpp(176): error C2678: binary '==': no operator found which takes a left-hand operand of type 'Catch::ExprLhs<spdlog::wmemory_buf_t &>' (or there is no acceptable conversion) [C:\projects\spdlog\build\tests\spdlog-utests.vcxproj]

...

C:\projects\spdlog\tests\test_misc.cpp(176): note: while trying to match the argument list '(Catch::ExprLhs<spdlog::wmemory_buf_t &>, const wchar_t [1])'

This comparison is not in the code. Let's rerun tests with the new PR and see if it helps resolve this too.