libstdc++: Fix use-after-free in std::format [PR119671]

libstdc++: Fix use-after-free in std::format [PR119671]

When formatting floating-point values to wide strings there's a case
where we invalidate a std::wstring buffer while a std::wstring_view is
still referring to it.

libstdc++-v3/ChangeLog:

	PR libstdc++/119671
	* include/std/format (__formatter_fp::format): Do not invalidate
	__wstr unless _M_localized returns a valid string.
	* testsuite/std/format/functions/format.cc: Check wide string
	formatting of floating-point types with classic locale.

Reviewed-by: Tomasz Kaminski <tkaminsk@redhat.com>
(cherry picked from commit e33b62eed7fd0a82d758b23252d288585b6790d2)