_M_fill initialization in basic_ios::init

[Paolo Carlini]

Thanks Nathan for your authoritative comments.

> If the facet's not there, the library can't do what you asked
> it to do.  Probably it should set failbit in that case, instead
> of crashing, though.

So, I could work on this in the next days: trying to avoid as much as possible
seg-faults in such cases (we have PR3983 too), setting failbit instead. Is
this your final word? Are there any other options in addition to
segfault/failbit ?

Thanks,
Paolo.