[PATCH] Fix crashes with mt_allocator if libstdc++ is dlclosed (PR libstdc++/22309, take 2)

[Paolo Carlini]

Hi Jakub,

>My previous patch fixed the original testcase in PR libstdc++/22309,
>but does not fix the testcase in comment #5.
>
Just wanted to make clear that I'm following your work on mt_allocator,
and appreciate a lot your help for this nasty issue. During the next
days, while waiting for Benjamin to also review it, I'm going to look
into it in better detail. Only minor comment, for now: instead of using
the __attribute__((__unused__)) extension, in C++ we can as well leave
the parameter unnamed (unless I'm missing something, sorry in that case).

Paolo.