[PATCH] libio: Eliminate _IO_stdin, _IO_stdout, _IO_stderr

[Florian Weimer]

* Dmitry V. Levin:

> Looks like _IO_legacy_file makes sense only when &_IO_stdin_used == NULL.
> If the check was moved inside _IO_legacy_file, then ...

I'm not sure.  We have seen cases where new binaries do not define
_IO_stdin_used, perhaps related to symbol visibility.

<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=634261>
<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=816059>
<https://bugs.launchpad.net/ubuntu/+source/lua5.3/+bug/1570055>

So at least for the free call (in _IO_deallocate_file after the patch),
I really want to check that the object isn't any of the preallocated
ones, in case the crashes aren't immediate and we have the potential
here for causing heap corruption.

Thanks,
Florian