libstdc++: Fix std::to_chars buffer overflow (PR 95851)

libstdc++: Fix std::to_chars buffer overflow (PR 95851)

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.