[PATCH] setipv4sourcefilter: Avoid using alloca.

[Florian Weimer]

* Siddhesh Poyarekar:

> scratch_buffer_free will also likely tamper with errno (it calls free
> after all) so it might make sense to save/restore errno here.  In fact
> I wonder if it makes sense to have scratch_buffer_free do that so that
> it's always safe to use it without worrying about errno.

We need to change free not to clobber errno.  Mainly this requires
protecting munmap and mprotect calls.  It's a QoI issue.

Thanks,
Florian