[PATCH] libio: Free backup area when it not required (BZ#22415)

[Florian Weimer]

On 11/29/2017 02:39 PM, Adhemerval Zanella wrote:
> Some libio operations fail to correctly free the backup area (created
> by_IO_{w}default_pbackfail on unget{w}c) resulting in either invalid
> buffer free operations or memory leaks.
> 
> For instance, on the example provided by BZ#22415 a following
> fputc after a fseek to rewind the stream issues an invalid free on
> the buffer.

Do we have to treat this as a security bug?  I don't think so, but I'd 
still like a second opinion.

Thanks,
Florian