[v3, 3.4] Fix libstdc++/12077

[Mark Mitchell]

Paolo Carlini wrote:

> Hi,
>
> sorry: it occurred to me only today that fixing this bug requires adding
> a member to class stdio_sync_filebuf.
>
> Therefore, if nobody among the libstdc++ maintainers and contributors
> disagree, I have to ask Mark permission for 3.4.0 too.
>
> Actually, the patch is quite straighforward, IMO, since is a simplified
> version of what is already in basic_filebuf.
>
> Tested x86-linux. 

OK for 3.4.0, in the name of forward ABI compatibility...

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Mark Mitchell
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