[PATCH v2] mbstowcs: Document, test, and fix null pointer dst semantics (Bug 25219)

[Florian Weimer]

* Carlos O'Donell:

> How about this?
> ~~~
> This behaviour of accepting a null pointer for @var{wstring} is an @w{XPG4.2}
> extension that is not specified in @w{ISO C} and is optional in @w{POSIX}.
> ~~~

Looks good to me.