[Bug libstdc++/70893] codecvt incorrectly decodes UTF-16

[kirillnow at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70893

--- Comment #1 from Кирилл <kirillnow at gmail dot com> ---
Bad guess on my part, sorry!
Actual problem is:
305:    else if (is_low_surrogate(c))
306:      return invalid_mb_sequence;
Stand-alone low surrogates are not uncommon, and could be decoded as valid
utf-8.
Example: Thorn  U+00DE Þ Encoded by [iconv (GNU libc) 2.23] -> c=0xDE00