The __detail::__to_chars_2 function assumes it won't be called with zero
values. However, when the output buffer is empty the caller doesn't
handle zero values correctly, and calls __to_chars_2 with a zero value,
resulting in an overflow of the empty buffer.
The __detail::__to_chars_i function should just return immediately for
an empty buffer, and otherwise ensure zero values are handled properly.
libstdc++-v3/ChangeLog:
PR libstdc++/95851
* include/std/charconv (__to_chars_i): Check for zero-sized
buffer unconditionally.
* testsuite/20_util/to_chars/95851.cc: New test.
(cherry picked from commit be50843754b4c4d47f0d628a84b3dbf2a4145a43)
