[Patch] Fix libstdc++/12352

[Paolo Carlini]

Pétur Runólfsson wrote:

>Gabriel Dos Reis wrote:
>  
>
>>| Tested x86-linux. Do you like it?
>>
>>It certainly is an improvement.  Thanks.
>>I was wondering whether that should not just use the traditional RAII
>>in form of objects construction/destruction (a la sentry), instead of
>>doing it manually.... Just thinking loudly...
>>    
>>
>I agree.
>
>In any case, this hunk isn't needed, because there is nothing in
>the try block that can throw:
>
Thanks for your reviews.

Therefore, if nobody objects, for the time being I will test and commit 
a variant of what I have posted (without the silly hunk for the 
placement new allocations, thanks Pétur!) to close the PR, then will 
look into more radical changes...

Paolo.