libstdc++/9828: Regression: Segmentation fault in num_put::put

[Pétur Runólfsson]

Jerry Quinn wrote:
> Another is that locales carry the cache info to avoid this problem.

I think this might be the best solution; it is very common to
create a stringstream just to format one number (see for
example boost::lexical_cast or operator<<(ostream&, complex&)).
Storing the cache in the locale would allow such code to take
advantage of the caches.

It should be easy to implement this using the same techniques
as are used for storing facets in locales.

Regards,
Petur